diff --git a/src/MatrixElement.cc b/src/MatrixElement.cc
index 3236a13..9f1c0ba 100644
--- a/src/MatrixElement.cc
+++ b/src/MatrixElement.cc
@@ -1,2473 +1,2473 @@
 /**
  *  \authors   The HEJ collaboration (see AUTHORS for details)
  *  \date      2019-2022
  *  \copyright GPLv2 or later
  */
 #include "HEJ/MatrixElement.hh"
 
 #include <algorithm>
 #include <cassert>
 #include <cmath>
 #include <cstddef>
 #include <cstdlib>
 #include <iterator>
 #include <limits>
 #include <unordered_map>
 #include <utility>
 
 #include "fastjet/PseudoJet.hh"
 
 #include "HEJ/ConfigFlags.hh"
 #include "HEJ/Constants.hh"
 #include "HEJ/EWConstants.hh"
 #include "HEJ/Event.hh"
 #include "HEJ/HiggsCouplingSettings.hh"
 #include "HEJ/Hjets.hh"
 #include "HEJ/LorentzVector.hh"
 #include "HEJ/PDG_codes.hh"
 #include "HEJ/Particle.hh"
 #include "HEJ/WWjets.hh"
 #include "HEJ/Wjets.hh"
 #include "HEJ/Zjets.hh"
 #include "HEJ/event_types.hh"
 #include "HEJ/exceptions.hh"
 #include "HEJ/jets.hh"
 #include "HEJ/utility.hh"
 
 namespace HEJ {
   namespace {
 
     // Colour acceleration multiplier for gluons
     // see eq:K_g in developer manual
     double K_g(double p1minus, double paminus) {
       return 1./2.*(p1minus/paminus + paminus/p1minus)*(C_A - 1./C_A) + 1./C_A;
     }
     double K_g(
       CLHEP::HepLorentzVector const & pout,
       CLHEP::HepLorentzVector const & pin
     ) {
       if(pin.z() > 0) return K_g(pout.plus(), pin.plus());
       return K_g(pout.minus(), pin.minus());
     }
 
     // Colour acceleration multiplier for quarks
     // see eq:K_q in developer manual
     constexpr double K_q = C_F;
 
     // Colour acceleration multipliers
     double K(
         ParticleID type,
         CLHEP::HepLorentzVector const & pout,
         CLHEP::HepLorentzVector const & pin
     ){
       if(type == pid::gluon) return K_g(pout, pin);
       return K_q;
     }
   } // namespace
 
   double MatrixElement::omega0(
       double alpha_s, double mur,
       fastjet::PseudoJet const & q_j
   ) const {
     const double lambda = param_.regulator_lambda;
     const double result = - alpha_s*N_C/M_PI*std::log(q_j.perp2()/(lambda*lambda));
     if(! param_.log_correction) return result;
     return (
         1. + alpha_s/(4.*M_PI)*BETA0*std::log(mur*mur/(q_j.perp()*lambda))
     )*result;
   }
 
   Weights MatrixElement::operator()(Event const & event) const {
     std::vector <double> tree_kin_part=tree_kin(event);
     std::vector <Weights> virtual_part=virtual_corrections(event);
     if(tree_kin_part.size() != virtual_part.size()) {
       throw std::logic_error("tree and virtuals have different sizes");
     }
     Weights sum = Weights{0., std::vector<double>(event.variations().size(), 0.)};
     for(size_t i=0; i<tree_kin_part.size(); ++i) {
       sum += tree_kin_part.at(i)*virtual_part.at(i);
     }
     return tree_param(event)*sum;
   }
 
   Weights MatrixElement::tree(Event const & event) const {
     std::vector <double> tree_kin_part=tree_kin(event);
     double sum = 0.;
     for(double i : tree_kin_part) {
       sum += i;
     }
     return tree_param(event)*sum;
   }
 
   Weights MatrixElement::tree_param(Event const & event) const {
     if(! is_resummable(event.type())) {
       return Weights{0., std::vector<double>(event.variations().size(), 0.)};
     }
     Weights result;
     // only compute once for each renormalisation scale
     std::unordered_map<double, double> known;
     result.central = tree_param(event, event.central().mur);
     known.emplace(event.central().mur, result.central);
     for(auto const & var: event.variations()) {
       const auto ME_it = known.find(var.mur);
       if(ME_it == end(known)) {
         const double wt = tree_param(event, var.mur);
         result.variations.emplace_back(wt);
         known.emplace(var.mur, wt);
       }
       else {
         result.variations.emplace_back(ME_it->second);
       }
     }
     return result;
   }
 
   std::vector<Weights> MatrixElement::virtual_corrections(Event const & event) const {
     if(!event.valid_hej_state(param_.soft_pt_regulator)) {
       return {Weights{0., std::vector<double>(event.variations().size(), 0.)}};
     }
     // only compute once for each renormalisation scale
     std::unordered_map<double, std::vector<double> > known_vec;
     std::vector<double> central_vec=virtual_corrections(event, event.central().mur);
     known_vec.emplace(event.central().mur, central_vec);
     for(auto const & var: event.variations()) {
       const auto ME_it = known_vec.find(var.mur);
       if(ME_it == end(known_vec)) {
         known_vec.emplace(var.mur, virtual_corrections(event, var.mur));
       }
     }
     // At this stage known_vec contains one vector of virtual corrections for each mur value
     // Now put this into a vector of Weights
     std::vector<Weights> result_vec;
     for(size_t i=0; i<central_vec.size(); ++i) {
       Weights result;
       result.central = central_vec.at(i);
       for(auto const & var: event.variations()) {
         const auto ME_it = known_vec.find(var.mur);
         result.variations.emplace_back(ME_it->second.at(i));
       }
       result_vec.emplace_back(result);
     }
     return result_vec;
   }
 
   template<class InputIterator>
   std::vector <double> MatrixElement::virtual_corrections_interference(
       InputIterator begin_parton, InputIterator end_parton,
       fastjet::PseudoJet const & q0_t,
       fastjet::PseudoJet const & q0_b,
       const double mur
   ) const{
     const double alpha_s = alpha_s_(mur);
 
     auto qi_t = q0_t;
     auto qi_b = q0_b;
 
     double sum_top = 0.;
     double sum_bot = 0.;
     double sum_mix = 0.;
     for(auto parton_it = begin_parton; parton_it != end_parton; ++parton_it){
       Particle parton = *parton_it;
       Particle parton_next = *(parton_it+1);
       const double dy = parton_next.rapidity() - parton.rapidity();
       const double tmp_top = omega0(alpha_s, mur, qi_t)*dy;
       const double tmp_bot = omega0(alpha_s, mur, qi_b)*dy;
       sum_top += tmp_top;
       sum_bot += tmp_bot;
       sum_mix += (tmp_top + tmp_bot) / 2.;
       qi_t -= parton_next.p;
       qi_b -= parton_next.p;
     }
 
     if (param_.nlo.enabled){
         return {(sum_top), (sum_bot), (sum_mix)};
     }
 
     return {exp(sum_top), exp(sum_bot), exp(sum_mix)};
 
 
   }
 
   double MatrixElement::virtual_corrections_W(
       Event const & event,
       const double mur,
       Particle const & WBoson
   ) const{
     auto const & in = event.incoming();
     const auto partons = filter_partons(event.outgoing());
     fastjet::PseudoJet const & pa = in.front().p;
 #ifndef NDEBUG
     fastjet::PseudoJet const & pb = in.back().p;
     double const norm = (in.front().p + in.back().p).E();
 #endif
 
     assert(std::is_sorted(partons.begin(), partons.end(), rapidity_less{}));
     assert(partons.size() >= 2);
     assert(pa.pz() < pb.pz());
 
     fastjet::PseudoJet q = pa - partons[0].p;
     std::size_t first_idx = 0;
     std::size_t last_idx = partons.size() - 1;
 
 #ifndef NDEBUG
     bool wc = true;
 #endif
     bool wqq = false;
 
     // With extremal qqbar or unordered gluon outside the extremal
     // partons then it is not part of the FKL ladder and does not
     // contribute to the virtual corrections. W emitted from the
     // most backward leg must be taken into account in t-channel
 
     if (event.type() == event_type::unob) {
       q -= partons[1].p;
       ++first_idx;
       if (in[0].type != partons[1].type ){
         q -= WBoson.p;
 #ifndef NDEBUG
         wc=false;
 #endif
       }
     }
 
     else if (event.type() == event_type::qqbar_exb) {
       q -= partons[1].p;
       ++first_idx;
       if (std::abs(partons[0].type) != std::abs(partons[1].type)){
         q -= WBoson.p;
 #ifndef NDEBUG
         wc=false;
 #endif
       }
     }
     else {
       if(event.type() == event_type::unof
          || event.type() == event_type::qqbar_exf){
         --last_idx;
       }
       if (in[0].type != partons[0].type ){
         q -= WBoson.p;
 #ifndef NDEBUG
         wc=false;
 #endif
       }
     }
 
 
     std::size_t first_idx_qqbar = last_idx;
     std::size_t last_idx_qqbar = last_idx;
 
     //if qqbarMid event, virtual correction do not occur between qqbar pair.
     if(event.type() == event_type::qqbar_mid){
       const auto backquark = std::find_if(
         begin(partons) + 1, end(partons) - 1 ,
         [](Particle const & s){ return (s.type != pid::gluon); }
       );
       if(backquark == end(partons) || (backquark+1)->type==pid::gluon) return 0;
       if(std::abs(backquark->type) != std::abs((backquark+1)->type)) {
         wqq=true;
 #ifndef NDEBUG
         wc=false;
 #endif
       }
       last_idx = std::distance(begin(partons), backquark);
       first_idx_qqbar = last_idx+1;
     }
     double exponent = 0;
     const double alpha_s = alpha_s_(mur);
     for(std::size_t j = first_idx; j < last_idx; ++j){
       exponent += omega0(alpha_s, mur, q)*(
           partons[j+1].rapidity() - partons[j].rapidity()
       );
       q -=partons[j+1].p;
     } // End Loop one
 
     if (last_idx != first_idx_qqbar) q -= partons[last_idx+1].p;
     if (wqq)  q -= WBoson.p;
 
     for(std::size_t j = first_idx_qqbar; j < last_idx_qqbar; ++j){
       exponent += omega0(alpha_s, mur, q)*(
           partons[j+1].rapidity() - partons[j].rapidity()
       );
       q -= partons[j+1].p;
     }
 
 #ifndef NDEBUG
     if (wc) q -= WBoson.p;
     assert(
         nearby(q, -1*pb, norm)
         || is_AWZH_boson(partons.back().type)
         || event.type() == event_type::unof
         || event.type() == event_type::qqbar_exf
     );
 #endif
     if (param_.nlo.enabled){
         double nlo_virtual =1.;
         //Only apply virtual corrections to a nlo order event.
         if (partons.size() == param_.nlo.nj) nlo_virtual+=exponent;
         return nlo_virtual;
     }
     return std::exp(exponent);
   }
 
 
 
   std::vector <double> MatrixElement::virtual_corrections_WW(
       Event const & event,
       const double mur
   ) const{
     auto const & in = event.incoming();
     const auto partons = filter_partons(event.outgoing());
     fastjet::PseudoJet const & pa = in.front().p;
 #ifndef NDEBUG
     fastjet::PseudoJet const & pb = in.back().p;
 #endif
 
     assert(std::is_sorted(partons.begin(), partons.end(), rapidity_less{}));
     assert(partons.size() >= 2);
     assert(pa.pz() < pb.pz());
     assert(event.decays().size() == 2);
 
     std::vector<fastjet::PseudoJet> plbar;
     std::vector<fastjet::PseudoJet> pl;
 
     for(auto const & decay_pair : event.decays()) {
       auto const decay = decay_pair.second;
       if(decay.at(0).type < 0) {
         plbar.emplace_back(decay.at(0).p);
         pl   .emplace_back(decay.at(1).p);
       }
       else {
         pl   .emplace_back(decay.at(0).p);
         plbar.emplace_back(decay.at(1).p);
       }
     }
 
     fastjet::PseudoJet q_t = pa - partons[0].p - pl[0] - plbar[0];
     fastjet::PseudoJet q_b = pa - partons[0].p - pl[1] - plbar[1];
 
     auto const begin_parton = cbegin(partons);
     auto const end_parton = cend(partons) - 1;
 
     if (param_.nlo.enabled){
         std::vector<double> virt_corrections_nlo {1.0,1.0,1.0}; //set virtual corrections to 1.
 
         // Only apply virtual corrections to a nlo order event.
         if (partons.size() == param_.nlo.nj) {
             std::vector <double> virt_corrections_nlo_interference =
               virtual_corrections_interference(
                 begin_parton, end_parton, q_t, q_b, mur
               );
             assert(
               virt_corrections_nlo_interference.size()
               == virt_corrections_nlo.size()
             );
 
             for(std::size_t i = 0; i < virt_corrections_nlo.size(); ++i){
                 virt_corrections_nlo[i] += virt_corrections_nlo_interference[i];
             }
         }
         return virt_corrections_nlo;
     }
 
     return virtual_corrections_interference(begin_parton, end_parton, q_t, q_b, mur);
 
   }
 
   std::vector <double> MatrixElement::virtual_corrections_Z_qq(
       Event const & event,
       const double mur,
       Particle const & ZBoson
   ) const{
     auto const & in = event.incoming();
     const auto partons = filter_partons(event.outgoing());
     fastjet::PseudoJet const & pa = in.front().p;
 #ifndef NDEBUG
     fastjet::PseudoJet const & pb = in.back().p;
 #endif
 
     assert(std::is_sorted(partons.begin(), partons.end(), rapidity_less{}));
     assert(partons.size() >= 2);
     assert(pa.pz() < pb.pz());
 
     fastjet::PseudoJet q_t = pa - partons[0].p - ZBoson.p;
     fastjet::PseudoJet q_b = pa - partons[0].p;
 
     auto begin_parton = cbegin(partons);
     auto end_parton = cend(partons) - 1;
 
     // Unordered gluon does not contribute to the virtual corrections
     if (event.type() == event_type::unob) {
       // Gluon is partons[0] and is already subtracted
       // partons[1] is the backward quark
       q_t -= partons[1].p;
       q_b -= partons[1].p;
       ++begin_parton;
     } else if (event.type() == event_type::unof) {
       // End sum at forward quark
       --end_parton;
     }
 
     if (param_.nlo.enabled){
 
         //set virtual corrections to 1.
         std::vector<double> virt_corrections_nlo {1.0,1.0,1.0};
 
         // Only apply virtual corrections to a nlo order event.
         if (partons.size() == param_.nlo.nj) {
             std::vector <double> virt_corrections_nlo_interference =
               virtual_corrections_interference(
                 begin_parton, end_parton, q_t, q_b, mur
               );
             assert(
               virt_corrections_nlo.size()
               == virt_corrections_nlo_interference.size()
             );
             for(std::size_t i = 0; i < virt_corrections_nlo.size(); ++i){
                 virt_corrections_nlo[i] += virt_corrections_nlo_interference[i];
             }
         }
         return virt_corrections_nlo;
     }
 
     return virtual_corrections_interference(begin_parton, end_parton, q_t, q_b, mur);
   }
 
   double MatrixElement::virtual_corrections_Z_qg(
       Event const & event,
       const double mur,
       Particle const & ZBoson,
       const bool is_gq_event
   ) const{
     auto const & in = event.incoming();
     const auto partons = filter_partons(event.outgoing());
     fastjet::PseudoJet const & pa = in.front().p;
 #ifndef NDEBUG
     fastjet::PseudoJet const & pb = in.back().p;
 #endif
 
     assert(std::is_sorted(partons.begin(), partons.end(), rapidity_less{}));
     assert(partons.size() >= 2);
     assert(pa.pz() < pb.pz());
 
     // If this is a gq event, don't subtract the Z momentum from first q
     fastjet::PseudoJet q = (is_gq_event ? pa - partons[0].p : pa - partons[0].p - ZBoson.p);
     size_t first_idx = 0;
     size_t last_idx = partons.size() - 1;
 
     // Unordered gluon does not contribute to the virtual corrections
     if (event.type() == event_type::unob) {
       // Gluon is partons[0] and is already subtracted
       // partons[1] is the backward quark
       q -= partons[1].p;
       ++first_idx;
     } else if (event.type() == event_type::unof) {
       // End sum at forward quark
       --last_idx;
     }
 
     double sum=0.;
     const double alpha_s = alpha_s_(mur);
     for(size_t j = first_idx; j < last_idx; ++j){
       sum += omega0(alpha_s, mur, q)*(partons[j+1].rapidity()
                                       - partons[j].rapidity());
       q -= partons[j+1].p;
     }
 
     if (param_.nlo.enabled){
         double nlo_virtual =1.;
         //Only apply virtual corrections to a nlo order event.
         if (partons.size() == param_.nlo.nj) nlo_virtual+=sum;
         return nlo_virtual;
     }
     return exp(sum);
   }
 
   std::vector<double> MatrixElement::virtual_corrections(
       Event const & event,
       const double mur
   ) const{
     auto const & in = event.incoming();
     auto const & out = event.outgoing();
     fastjet::PseudoJet const & pa = in.front().p;
 #ifndef NDEBUG
     fastjet::PseudoJet const & pb = in.back().p;
     double const norm = (in.front().p + in.back().p).E();
 #endif
 
     std::vector<Particle> bosons = filter_AWZH_bosons(out);
 
     if(event.jets().size() != param_.nlo.nj && param_.nlo.enabled) {
         throw std::logic_error{
           "Input event has number of jets different to stated NLO "
           "input in config file: " + std::to_string(event.jets().size())
           + " vs "  +std::to_string(param_.nlo.nj) + "\n"
         };
     }
     if(bosons.size() > 2) {
       throw not_implemented("Emission of >2 bosons is unsupported");
     }
 
     if(bosons.size() == 2) {
       if(bosons[0].type == pid::Wp && bosons[1].type == pid::Wp) {
         return virtual_corrections_WW(event, mur);
       }
       else if(bosons[0].type == pid::Wm && bosons[1].type == pid::Wm) {
         return virtual_corrections_WW(event, mur);
       }
       throw not_implemented("Emission of bosons of unsupported type");
     }
 
     if(bosons.size() == 1) {
       const auto AWZH_boson = bosons[0];
 
       if(std::abs(AWZH_boson.type) == pid::Wp){
         return {virtual_corrections_W(event, mur, AWZH_boson)};
       }
 
       if(AWZH_boson.type == pid::Z_photon_mix){
         if(is_gluon(in.back().type)){
           // This is a qg event
           return {virtual_corrections_Z_qg(event, mur, AWZH_boson, false)};
         }
         if(is_gluon(in.front().type)){
           // This is a gq event
           return {virtual_corrections_Z_qg(event, mur, AWZH_boson, true)};
         }
         // This is a qq event
         return virtual_corrections_Z_qq(event, mur, AWZH_boson);
       }
     }
 
 
     assert(std::is_sorted(out.begin(), out.end(), rapidity_less{}));
     assert(out.size() >= 2);
     assert(pa.pz() < pb.pz());
 
     fastjet::PseudoJet q = pa - out[0].p;
     std::size_t first_idx = 0;
     std::size_t last_idx = out.size() - 1;
 
     // if there is a Higgs boson _not_ emitted off an incoming gluon,
     // extremal qqbar or unordered gluon outside the extremal partons
     // then it is not part of the FKL ladder
     // and does not contribute to the virtual corrections
     if((out.front().type == pid::Higgs && in.front().type != pid::gluon)
        || event.type() == event_type::unob
        || event.type() == event_type::qqbar_exb){
       q -= out[1].p;
       ++first_idx;
     }
     if((out.back().type == pid::Higgs && in.back().type != pid::gluon)
        || event.type() == event_type::unof
        || event.type() == event_type::qqbar_exf){
       --last_idx;
     }
 
     std::size_t first_idx_qqbar = last_idx;
     std::size_t last_idx_qqbar = last_idx;
 
     //if central qqbar event, virtual correction do not occur between q-qbar.
     if(event.type() == event_type::qqbar_mid){
       const auto backquark = std::find_if(
         begin(out) + 1, end(out) - 1 ,
         [](Particle const & s){ return (s.type != pid::gluon && is_parton(s.type)); }
       );
       if(backquark == end(out) || (backquark+1)->type==pid::gluon) return {0.};
       last_idx = std::distance(begin(out), backquark);
       first_idx_qqbar = last_idx+1;
     }
     double exponent = 0;
     const double alpha_s = alpha_s_(mur);
     for(std::size_t j = first_idx; j < last_idx; ++j){
       exponent += omega0(alpha_s, mur, q)*(
           out[j+1].rapidity() - out[j].rapidity()
       );
       q -= out[j+1].p;
     }
 
     if (last_idx != first_idx_qqbar) q -= out[last_idx+1].p;
 
     for(std::size_t j = first_idx_qqbar; j < last_idx_qqbar; ++j){
       exponent += omega0(alpha_s, mur, q)*(
           out[j+1].rapidity() - out[j].rapidity()
       );
       q -= out[j+1].p;
     }
     assert(
         nearby(q, -1*pb, norm)
         || (out.back().type == pid::Higgs && in.back().type != pid::gluon)
         || event.type() == event_type::unof
         || event.type() == event_type::qqbar_exf
     );
 
     if (param_.nlo.enabled){
         const auto partons = filter_partons(event.outgoing());
         double nlo_virtual =1.;
         //Only apply virtual corrections to a nlo order event.
         if (partons.size() == param_.nlo.nj) nlo_virtual+=exponent;
         return {nlo_virtual};
     }
     return {std::exp(exponent)};
   }
 
 namespace {
 
   //! Lipatov vertex for partons emitted into extremal jets
   CLHEP::HepLorentzVector CLipatov(
       CLHEP::HepLorentzVector const & qav, CLHEP::HepLorentzVector const & qbv,
       CLHEP::HepLorentzVector const & p1, CLHEP::HepLorentzVector const & p2
   ) {
     const CLHEP::HepLorentzVector p5 = qav-qbv;
     const CLHEP::HepLorentzVector CL = -(qav+qbv)
       + p1*(qav.m2()/p5.dot(p1) + 2.*p5.dot(p2)/p1.dot(p2))
       - p2*(qbv.m2()/p5.dot(p2) + 2.*p5.dot(p1)/p1.dot(p2));
     return CL;
   }
 
   double C2Lipatov(
       CLHEP::HepLorentzVector const & qav,
       CLHEP::HepLorentzVector const & qbv,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & p2
   ){
     const CLHEP::HepLorentzVector CL = CLipatov(qav, qbv, p1, p2);
     return -CL.dot(CL);
   }
 
   //! Lipatov vertex with soft subtraction for partons emitted into extremal jets
   double C2Lipatovots(
       CLHEP::HepLorentzVector const & qav,
       CLHEP::HepLorentzVector const & qbv,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & p2,
       const double lambda
   ) {
     const double Cls=(C2Lipatov(qav, qbv, p1, p2)/(qav.m2()*qbv.m2()));
 
     const double kperp=(qav-qbv).perp();
     if (kperp>lambda)
       return Cls;
 
     return Cls-4./(kperp*kperp);
   }
 
   double C2Lipatov_Mix(
       CLHEP::HepLorentzVector const & qav_t, CLHEP::HepLorentzVector const & qbv_t,
       CLHEP::HepLorentzVector const & qav_b, CLHEP::HepLorentzVector const & qbv_b,
       CLHEP::HepLorentzVector const & p1, CLHEP::HepLorentzVector const & p2
   ) {
     const CLHEP::HepLorentzVector CL_t = CLipatov(qav_t, qbv_t, p1, p2);
     const CLHEP::HepLorentzVector CL_b = CLipatov(qav_b, qbv_b, p1, p2);
     return -CL_t.dot(CL_b);
   }
 
   double C2Lipatovots_Mix(
       CLHEP::HepLorentzVector const & qav_t, CLHEP::HepLorentzVector const & qbv_t,
       CLHEP::HepLorentzVector const & qav_b, CLHEP::HepLorentzVector const & qbv_b,
       CLHEP::HepLorentzVector const & p1, CLHEP::HepLorentzVector const & p2,
       const double lambda
   ) {
     const double Cls = C2Lipatov_Mix(qav_t, qbv_t, qav_b, qbv_b, p1, p2)
                        / sqrt(qav_t.m2() * qbv_t.m2() * qav_b.m2() * qbv_b.m2());
     const double kperp = (qav_t - qbv_t).perp();
     if (kperp > lambda){
       return Cls;
     }
     return Cls - 4.0 / (kperp * kperp);
 
   }
 
   CLHEP::HepLorentzVector CLipatov(
       CLHEP::HepLorentzVector const & qav, CLHEP::HepLorentzVector const & qbv,
       CLHEP::HepLorentzVector const & pim, CLHEP::HepLorentzVector const & pip,
       CLHEP::HepLorentzVector const & pom, CLHEP::HepLorentzVector const & pop
   ){
     const CLHEP::HepLorentzVector p5 = qav-qbv;
     const CLHEP::HepLorentzVector CL = -(qav+qbv)
       + qav.m2()*(1./p5.dot(pip)*pip + 1./p5.dot(pop)*pop)/2.
       - qbv.m2()*(1./p5.dot(pim)*pim + 1./p5.dot(pom)*pom)/2.
       + ( pip*(p5.dot(pim)/pip.dot(pim) + p5.dot(pom)/pip.dot(pom))
           + pop*(p5.dot(pim)/pop.dot(pim) + p5.dot(pom)/pop.dot(pom))
           - pim*(p5.dot(pip)/pip.dot(pim) + p5.dot(pop)/pop.dot(pim))
           - pom*(p5.dot(pip)/pip.dot(pom) + p5.dot(pop)/pop.dot(pom)) )/2.;
     return CL;
   }
 
   //! Lipatov vertex
   double C2Lipatov( // B
       CLHEP::HepLorentzVector const & qav,
       CLHEP::HepLorentzVector const & qbv,
       CLHEP::HepLorentzVector const & pim,
       CLHEP::HepLorentzVector const & pip,
       CLHEP::HepLorentzVector const & pom,
       CLHEP::HepLorentzVector const & pop
   ){
     const CLHEP::HepLorentzVector CL = CLipatov(qav, qbv, pim, pip, pom, pop);
     return -CL.dot(CL);
   }
 
   //! Lipatov vertex with soft subtraction
   double C2Lipatovots(
       CLHEP::HepLorentzVector const & qav,
       CLHEP::HepLorentzVector const & qbv,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & p2,
       const double lambda
   ) {
     const double Cls=(C2Lipatov(qav, qbv, pa, pb, p1, p2)/(qav.m2()*qbv.m2()));
 
     const double kperp=(qav-qbv).perp();
     if (kperp>lambda)
       return Cls;
     return Cls-4./(kperp*kperp);
   }
 
   double C2Lipatov_Mix(
       CLHEP::HepLorentzVector const & qav_t, CLHEP::HepLorentzVector const & qbv_t,
       CLHEP::HepLorentzVector const & qav_b, CLHEP::HepLorentzVector const & qbv_b,
       CLHEP::HepLorentzVector const & pim, CLHEP::HepLorentzVector const & pip,
       CLHEP::HepLorentzVector const & pom, CLHEP::HepLorentzVector const & pop
   ) {
     const CLHEP::HepLorentzVector CL_t = CLipatov(qav_t, qbv_t, pim, pip, pom, pop);
     const CLHEP::HepLorentzVector CL_b = CLipatov(qav_b, qbv_b, pim, pip, pom, pop);
     return -CL_t.dot(CL_b);
   }
 
   double C2Lipatovots_Mix(
       CLHEP::HepLorentzVector const & qav_t, CLHEP::HepLorentzVector const & qbv_t,
       CLHEP::HepLorentzVector const & qav_b, CLHEP::HepLorentzVector const & qbv_b,
       CLHEP::HepLorentzVector const & pa, CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1, CLHEP::HepLorentzVector const & p2,
       const double lambda
   ) {
     const double Cls = C2Lipatov_Mix(qav_t, qbv_t, qav_b, qbv_b, pa, pb, p1, p2)
                        / sqrt(qav_t.m2() * qbv_t.m2() * qav_b.m2() * qbv_b.m2());
     const double kperp = (qav_t - qbv_t).perp();
     if (kperp > lambda) {
       return Cls;
     }
     return Cls - 4.0 / (kperp * kperp);
 
   }
 
   /** Matrix element squared for tree-level current-current scattering
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pg              Unordered gluon momentum
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @returns               ME Squared for Tree-Level Current-Current Scattering
    *
    *  @note The unof contribution can be calculated by reversing the argument ordering.
    */
   double ME_uno_current(
       [[maybe_unused]] ParticleID aptype,
       ParticleID bptype,
       CLHEP::HepLorentzVector const & pg,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa
   ){
     assert(aptype!=pid::gluon); // aptype cannot be gluon
 
     const double t1 = (pa - p1 - pg).m2();
     const double t2 = (pb - pn).m2();
 
     return K_q * K(bptype, pn, pb)*currents::ME_unob_qq(pg, p1, pa, pn, pb) / (t1 * t2);
   }
 
   /** Matrix element squared for tree-level current-current scattering
    *  @param bptype   Particle b PDG ID
    *  @param pgin     Incoming gluon momentum
    *  @param p1       More backward (anti-)quark from splitting Momentum
    *  @param p2       Less backward (anti-)quark from splitting Momentum
    *  @param pn       Particle n Momentum
    *  @param pb       Particle b Momentum
    *  @returns        ME Squared for Tree-Level Current-Current Scattering
    *
    *  @note The forward qqbar contribution can be calculated by reversing the
    *        argument ordering.
    */
   double ME_qqbar_current(
       ParticleID bptype,
       CLHEP::HepLorentzVector const & pgin,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & p2,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb
   ){
     const double t1 = (pgin - p1 - p2).m2();
     const double t2 = (pn - pb).m2();
 
     return K(bptype, pn, pb)*currents::ME_qqbar_qg(pb, pgin, pn, p2, p1) / (t1 * t2);
   }
 
   /*  \brief Matrix element squared for central qqbar tree-level current-current
    *         scattering
    *
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param nabove          Number of gluons emitted before central qqbarpair
    *  @param nbelow          Number of gluons emitted after central qqbarpair
    *  @param pa              Initial state a Momentum
    *  @param pb              Initial state b Momentum
    *  @param pq              Final state qbar Momentum
    *  @param pqbar           Final state q Momentum
    *  @param partons         Vector of all outgoing partons
    *  @returns               ME Squared for qqbar_mid Tree-Level Current-Current Scattering
    */
   double ME_qqbar_mid_current(
       ParticleID aptype, ParticleID bptype, int nabove,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & pq,
       CLHEP::HepLorentzVector const & pqbar,
       std::vector<CLHEP::HepLorentzVector> const & partons
   ){
     using namespace currents;
     // CAM factors for the qqbar amps, and qqbar ordering (default, pq backwards)
     const bool swap_qqbar=pqbar.rapidity() < pq.rapidity();
 
     CLHEP::HepLorentzVector const & p1 = partons.front();
     CLHEP::HepLorentzVector const & pn = partons.back();
 
     const double t1 = (p1 - pa).m2();
     const double t2 = (pb - pn).m2();
 
     return K(aptype, p1, pa)
       *K(bptype, pn, pb)
       *ME_Cenqqbar_qq(
         pa, pb, partons,
         swap_qqbar, nabove
       ) / (t1 * t2);
   }
 
 
   /** Matrix element squared for tree-level current-current scattering
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @returns               ME Squared for Tree-Level Current-Current Scattering
    */
   double ME_current(
       ParticleID aptype, ParticleID bptype,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa
   ){
     const double t1 = (p1 - pa).m2();
     const double t2 = (pb - pn).m2();
 
     return K(aptype, p1, pa)
       * K(bptype, pn, pb)
       * currents::ME_qq(p1, pa, pn, pb)/(t1 * t2);
   }
 
   /** Matrix element squared for tree-level current-current scattering With W+Jets
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @param wc              Boolean. True->W Emitted from b. Else; emitted from leg a
    *  @returns               ME Squared for Tree-Level Current-Current Scattering
    */
   double ME_W_current(
       ParticleID aptype, ParticleID bptype,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & plbar,
       CLHEP::HepLorentzVector const & pl,
       bool const wc, ParticleProperties const & Wprop
   ){
     using namespace currents;
     assert(!(aptype==pid::gluon && bptype==pid::gluon));
 
     if(aptype == pid::gluon || wc) {
       // swap currents to ensure that the W is emitted off the first one
       return ME_W_current(bptype, aptype, p1, pa, pn, pb, plbar, pl, false, Wprop);
     }
 
     // we assume that the W is emitted off a quark line
     // if this is not the case, we have to apply CP conjugation,
     // which is equivalent to swapping lepton and antilepton momenta
     const double current_contr = is_quark(aptype)?
       ME_W_qQ(p1,plbar,pl,pa,pn,pb,Wprop):
       ME_W_qQ(p1,pl,plbar,pa,pn,pb,Wprop);
 
     const double t1 = (pa - p1 - pl - plbar).m2();
     const double tn = (pn - pb).m2();
 
     return K(aptype, p1, pa)
       * K(bptype, pn, pb)
       * current_contr/(4.*(N_C*N_C - 1) * t1 * tn);
   }
 
   /** Matrix element squared for backwards uno tree-level current-current
    *  scattering With W+Jets
    *
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @param pg              Unordered gluon momentum
    *  @param wc              Boolean. True->W Emitted from b. Else; emitted from leg a
    *  @returns               ME Squared for unob Tree-Level Current-Current Scattering
    *
    *  @note The unof contribution can be calculated by reversing the argument ordering.
    */
   double ME_W_uno_current(
       ParticleID aptype, ParticleID bptype,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & pg,
       CLHEP::HepLorentzVector plbar,
       CLHEP::HepLorentzVector pl,
       bool const wc, ParticleProperties const & Wprop
   ){
     using namespace currents;
     assert(bptype != pid::gluon || aptype != pid::gluon);
 
     if(aptype == pid::gluon || wc) {
       // emission off pb -- pn line
       // we assume that the W is emitted off a quark line
       // if this is not the case, we have to apply CP conjugation,
       // which is equivalent to swapping lepton and antilepton momenta
       if(is_antiquark(bptype)) std::swap(plbar, pl);
       const double t1 = (pa - p1 - pg).m2();
       const double tn = (pb - pn - plbar - pl).m2();
 
       return K_q*K_q*ME_W_unob_qQ(p1,pa,pn,pb,pg,plbar,pl,Wprop)/(4.*(N_C*N_C - 1) * t1 * tn);
     }
 
     // emission off pa -- p1 line
     if(is_antiquark(aptype)) std::swap(plbar, pl);
     const double t1 = (pa - p1 - pg - plbar - pl).m2();
     const double tn = (pb - pn).m2();
     return K(bptype, pn, pb)/C_F*ME_Wuno_qQ(p1,pa,pn,pb,pg,plbar,pl,Wprop)/(t1 * tn);
   }
 
   /** \brief Matrix element squared for backward qqbar tree-level current-current
    *         scattering With W+Jets
    *
    *  @param bptype          Particle b PDG ID
    *  @param pa              Initial state a Momentum
    *  @param pb              Initial state b Momentum
    *  @param pq              Final state q Momentum
    *  @param pqbar           Final state qbar Momentum
    *  @param pn              Final state n Momentum
    *  @param plbar           Final state anti-lepton momentum
    *  @param pl              Final state lepton momentum
    *  @param wc              Boolean. True->W Emitted from b. Else; emitted from leg a
    *  @returns               ME Squared for qqbarb Tree-Level Current-Current Scattering
    *
    *  @note calculate forwards qqbar contribution by reversing argument ordering.
    */
   double ME_W_qqbar_current(
       ParticleID bptype,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & pb,
-      CLHEP::HepLorentzVector pq,
-      CLHEP::HepLorentzVector pqbar,
+      CLHEP::HepLorentzVector const & pq,
+      CLHEP::HepLorentzVector const & pqbar,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & plbar,
       CLHEP::HepLorentzVector const & pl,
       bool const wc,
       ParticleProperties const & Wprop
   ){
     using namespace currents;
     if(is_anyquark(bptype) && wc) {
       // W Must be emitted from forwards leg.
       const double t1 = (pa - pq - pqbar).m2();
       const double tn = (pb - pn - pl - plbar).m2();
 
       return K_q*K_q*ME_W_Exqqbar_QQq(pb,pa,pn,pq,pqbar,plbar,pl,is_antiquark(bptype),Wprop)/(4.*(N_C*N_C - 1) * t1 * tn);
     }
 
     const double t1 = (pa - pl - plbar - pq - pqbar).m2();
     const double tn = (pb - pn).m2();
 
     return K(bptype, pn, pb)/C_F
       * ME_WExqqbar_qqbarQ(pa, pqbar, plbar, pl, pq, pn, pb, Wprop) / (t1 * tn);
   }
 
   /*  \brief Matrix element squared for central qqbar tree-level current-current
    *         scattering With W+Jets
    *
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param nabove          Number of gluons emitted before central qqbarpair
    *  @param nbelow          Number of gluons emitted after central qqbarpair
    *  @param pa              Initial state a Momentum
    *  @param pb              Initial state b Momentum\
    *  @param pq              Final state qbar Momentum
    *  @param pqbar           Final state q Momentum
    *  @param partons         Vector of all outgoing partons
    *  @param plbar           Final state anti-lepton momentum
    *  @param pl              Final state lepton momentum
    *  @param wqq             Boolean. True siginfies W boson is emitted from Central qqbar
    *  @param wc              Boolean. wc=true signifies w boson emitted from leg b; if wqq=false.
    *  @returns               ME Squared for qqbar_mid Tree-Level Current-Current Scattering
    */
   double ME_W_qqbar_mid_current(
       ParticleID aptype, ParticleID bptype,
       int nabove, int nbelow,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & pq,
       CLHEP::HepLorentzVector const & pqbar,
       std::vector<CLHEP::HepLorentzVector> const & partons,
       CLHEP::HepLorentzVector const & plbar,
       CLHEP::HepLorentzVector const & pl,
       bool const wqq, bool const wc,
       ParticleProperties const & Wprop
   ){
     using namespace currents;
     // CAM factors for the qqbar amps, and qqbar ordering (default, pq backwards)
     const bool swap_qqbar=pqbar.rapidity() < pq.rapidity();
     double wt=1.;
 
     if (aptype==pid::gluon) wt*=K_g(partons.front(),pa)/C_F;
     if (bptype==pid::gluon) wt*=K_g(partons.back(),pb)/C_F;
 
     if(wqq)
       return wt*ME_WCenqqbar_qq(pa, pb, pl, plbar, partons,
                               is_antiquark(aptype),is_antiquark(bptype),
                               swap_qqbar, nabove, Wprop);
     return wt*ME_W_Cenqqbar_qq(pa, pb, pl, plbar, partons,
                              is_antiquark(aptype), is_antiquark(bptype),
                              swap_qqbar, nabove, nbelow, wc, Wprop);
   }
 
   /** Matrix element squared for tree-level current-current scattering With Z+Jets
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @param plbar           Final state positron momentum
    *  @param pl              Final state electron momentum
    *  @param Zprop           Z properties
    *  @param stw2            Value of sin(theta_w)^2
    *  @param ctw             Value of cos(theta_w)
    *  @returns               ME Squared for Tree-Level Current-Current Scattering
    */
   std::vector<double> ME_Z_current(
       const ParticleID aptype, const ParticleID bptype,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & plbar,
       CLHEP::HepLorentzVector const & pl,
       ParticleProperties const & Zprop,
       const double stw2, const double ctw
   ){
     using namespace currents;
 
     const auto pZ = pl + plbar;
 
     const double pref = K(aptype, p1, pa) * K(bptype, pn, pb)/(4.*(N_C*N_C-1));
 
     // we know they are not both gluons
     assert(!is_gluon(aptype) || !is_gluon(bptype));
 
     if(is_anyquark(aptype) && is_gluon(bptype)){
       // This is a qg event
       const double t1 = (pa-p1-pZ).m2();
       const double tn = (pb-pn   ).m2();
       return { pref*ME_Z_qg(pa,pb,p1,pn,plbar,pl,aptype,bptype,Zprop,stw2,ctw)/(t1 * tn) };
     }
 
     if(is_gluon(aptype) && is_anyquark(bptype)){
       // This is a gq event
       const double t1 = (pa-p1   ).m2();
       const double tn = (pb-pn-pZ).m2();
       return { pref*ME_Z_qg(pb,pa,pn,p1,plbar,pl,bptype,aptype,Zprop,stw2,ctw)/(t1 * tn) };
     }
 
     // This is a qq event
     assert(is_anyquark(aptype) && is_anyquark(bptype));
     const double t1_top = (pa-p1-pZ).m2();
     const double t2_top = (pb-pn   ).m2();
 
     const double t1_bot = (pa-p1   ).m2();
     const double t2_bot = (pb-pn-pZ).m2();
     std::vector<double> res = ME_Z_qQ(pa,pb,p1,pn,plbar,pl,aptype,bptype,Zprop,stw2,ctw);
     assert(res.size() == 3);
 
     res[0] *= pref/(t1_top * t2_top);
     res[1] *= pref/(t1_bot * t2_bot);
     res[2] *= pref/sqrt(t1_top * t2_top * t1_bot * t2_bot);
 
     return res;
   }
 
   /** Matrix element squared for backwards uno tree-level current-current
    *  scattering With Z+Jets
    *
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @param pg              Unordered gluon momentum
    *  @param plbar           Final state positron momentum
    *  @param pl              Final state electron momentum
    *  @param Zprop           Z properties
    *  @param stw2            Value of sin(theta_w)^2
    *  @param ctw             Value of cos(theta_w)
    *  @returns               ME Squared for unob Tree-Level Current-Current Scattering
    *
    *  @note The unof contribution can be calculated by reversing the argument ordering.
    */
 
   std::vector<double> ME_Z_uno_current(
       const ParticleID aptype, const ParticleID bptype,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & pg,
       CLHEP::HepLorentzVector const & plbar,
       CLHEP::HepLorentzVector const & pl,
       ParticleProperties const & Zprop,
       const double stw2, const double ctw
   ){
     using namespace currents;
 
     const auto pZ = pl + plbar;
 
     const double pref = K(aptype, p1, pa)/C_F * K(bptype, pn, pb)/C_F;
 
     // we know they are not both gluons
     assert(!is_gluon(aptype) || !is_gluon(bptype));
 
     if (is_anyquark(aptype) && is_gluon(bptype)) {
       // This is a qg event
       const double t1 = (pa-p1-pg-pZ).m2();
       const double tn = (pb-pn      ).m2();
       return { pref*ME_Zuno_qg(pa,pb,pg,p1,pn,plbar,pl,aptype,bptype,Zprop,stw2,ctw)/(t1 * tn) };
     }
 
     if (is_gluon(aptype) && is_anyquark(bptype)) {
       // This is a gq event
       const double t1 = (pa-p1      ).m2();
       const double tn = (pb-pn-pg-pZ).m2();
       return { pref*ME_Zuno_qg(pb,pa,pg,pn,p1,plbar,pl,bptype,aptype,Zprop,stw2,ctw)/(t1 * tn) };
     }
 
     // This is a qq event
     assert(is_anyquark(aptype) && is_anyquark(bptype));
     const double t1_top = (pa-pg-p1-pZ).m2();
     const double t2_top = (pb-pn      ).m2();
 
     const double t1_bot = (pa-pg-p1).m2();
     const double t2_bot = (pb-pn-pZ).m2();
 
     std::vector<double> res = ME_Zuno_qQ(pa,pb,pg,p1,pn,plbar,pl,aptype,bptype,Zprop,stw2,ctw);
     assert(res.size() == 3);
 
     res[0] *= pref/(t1_top * t2_top);
     res[1] *= pref/(t1_bot * t2_bot);
     res[2] *= pref/sqrt(t1_top * t2_top * t1_bot * t2_bot);
 
     return res;
   }
 
   /** \brief Matrix element squared for tree-level current-current scattering with Higgs
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @param qH              t-channel momentum before Higgs
    *  @param qHp1            t-channel momentum after Higgs
    *  @returns               ME Squared for Tree-Level Current-Current Scattering with Higgs
    */
   double ME_Higgs_current(
       ParticleID aptype, ParticleID bptype,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & qH,  // t-channel momentum before Higgs
       CLHEP::HepLorentzVector const & qHp1, // t-channel momentum after Higgs
       double mt, bool include_bottom, double mb, double vev
   ){
     const double t1 = (pa - p1).m2();
     const double tn = (pb - pn).m2();
 
     return
       K(aptype, p1, pa)
       *K(bptype, pn, pb)
       *currents::ME_H_qQ(
         pn,pb,p1,pa,-qHp1,-qH,mt,include_bottom,mb,vev
       ) / (t1 * tn * qH.m2() * qHp1.m2());
   }
 
   /** \brief Current matrix element squared with Higgs and unordered backward emission
    *  @param aptype          Particle A PDG ID
    *  @param bptype          Particle B PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param pg              Unordered back Particle Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @param qH              t-channel momentum before Higgs
    *  @param qHp1            t-channel momentum after Higgs
    *  @returns               ME Squared with Higgs and unordered backward emission
    *
    *  @note This function assumes unordered gluon backwards from pa-p1 current.
    *        For unof, reverse call order
    */
   double ME_Higgs_current_uno(
       ParticleID aptype, ParticleID bptype,
       CLHEP::HepLorentzVector const & pg,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & qH,  // t-channel momentum before Higgs
       CLHEP::HepLorentzVector const & qHp1, // t-channel momentum after Higgs
       double mt, bool include_bottom, double mb, double vev
   ){
     const double t1 = (pa - p1 - pg).m2();
     const double tn = (pn - pb).m2();
 
     return
       K(aptype, p1, pa)
       *K(bptype, pn, pb)
       *currents::ME_H_unob_qQ(
         pg,p1,pa,pn,pb,-qH,-qHp1,mt,include_bottom,mb,vev
       ) / (t1 * qH.m2() * qHp1.m2() * tn);
   }
 
   /** Matrix element squared for tree-level scattering with WW+Jets
    *  @param aptype          Particle a PDG ID
    *  @param bptype          Particle b PDG ID
    *  @param pn              Particle n Momentum
    *  @param pb              Particle b Momentum
    *  @param p1              Particle 1 Momentum
    *  @param pa              Particle a Momentum
    *  @param pl1bar          Particle l1bar Momentum
    *  @param pl1             Particle l1 Momentum
    *  @param pl2bar          Particle l2bar Momentum
    *  @param pl2             Particle l2 Momentum
    *  @returns               ME Squared for Tree-Level Current-Current Scattering
    */
   std::vector <double> ME_WW_current(
       ParticleID aptype, ParticleID bptype,
       CLHEP::HepLorentzVector const & pn,
       CLHEP::HepLorentzVector const & pb,
       CLHEP::HepLorentzVector const & p1,
       CLHEP::HepLorentzVector const & pa,
       CLHEP::HepLorentzVector const & pl1bar,
       CLHEP::HepLorentzVector const & pl1,
       CLHEP::HepLorentzVector const & pl2bar,
       CLHEP::HepLorentzVector const & pl2,
       ParticleProperties const & Wprop
   ){
     using namespace currents;
 
     if (aptype > 0 && bptype > 0)
       return ME_WW_qQ(p1, pl1bar, pl1, pa, pn, pl2bar, pl2, pb, Wprop);
     if (aptype < 0 && bptype > 0)
       return ME_WW_qbarQ(p1, pl1bar, pl1, pa, pn, pl2bar, pl2, pb, Wprop);
     if (aptype > 0 && bptype < 0)
       return ME_WW_qQbar(p1, pl1bar, pl1, pa, pn, pl2bar, pl2, pb, Wprop);
     if (aptype < 0 && bptype < 0)
       return ME_WW_qbarQbar(p1, pl1bar, pl1, pa, pn, pl2bar, pl2, pb, Wprop);
 
     throw std::logic_error("unreachable");
   }
 
   void validate(MatrixElementConfig const & config) {
 #ifndef HEJ_BUILD_WITH_QCDLOOP
     if(!config.Higgs_coupling.use_impact_factors) {
       throw std::invalid_argument{
         "Invalid Higgs coupling settings.\n"
         "HEJ without QCDloop support can only use impact factors.\n"
         "Set use_impact_factors to true or recompile HEJ.\n"
        };
     }
 #endif
     if(config.Higgs_coupling.use_impact_factors
       && config.Higgs_coupling.mt != std::numeric_limits<double>::infinity()) {
       throw std::invalid_argument{
         "Conflicting settings: "
           "impact factors may only be used in the infinite top mass limit"
       };
     }
   }
 } // namespace
 
   MatrixElement::MatrixElement(
       std::function<double (double)> alpha_s,
       MatrixElementConfig conf
   ):
     alpha_s_{std::move(alpha_s)},
     param_{std::move(conf)}
   {
     validate(param_);
   }
 
   std::vector<double> MatrixElement::tree_kin(
       Event const & ev
   ) const {
     if(!ev.valid_hej_state(param_.soft_pt_regulator)) return {0.};
 
     if(!ev.valid_incoming()){
       throw std::invalid_argument{
         "Invalid momentum for one or more incoming particles. "
         "Incoming momenta must have vanishing mass and transverse momentum."
       };
     }
 
     std::vector<Particle> bosons = filter_AWZH_bosons(ev.outgoing());
 
     if(bosons.empty()) {
       return {tree_kin_jets(ev)};
     }
 
     if(bosons.size() == 1) {
       switch(bosons[0].type){
       case pid::Higgs:
         return {tree_kin_Higgs(ev)};
       case pid::Wp:
       case pid::Wm:
         return {tree_kin_W(ev)};
       case pid::Z_photon_mix:
         return tree_kin_Z(ev);
       // TODO
       case pid::photon:
       case pid::Z:
       default:
         throw not_implemented("Emission of boson of unsupported type");
       }
     }
 
     if(bosons.size() == 2) {
       if(bosons[0].type == pid::Wp && bosons[1].type == pid::Wp){
         return tree_kin_WW(ev);
       }
       else if(bosons[0].type == pid::Wm && bosons[1].type == pid::Wm){
         return tree_kin_WW(ev);
       }
 
       throw not_implemented("Emission of bosons of unsupported type");
     }
 
     throw not_implemented("Emission of >2 bosons is unsupported");
   }
 
   namespace {
     constexpr int EXTREMAL_JET_IDX = 1;
     constexpr int NO_EXTREMAL_JET_IDX = 0;
 
     bool treat_as_extremal(Particle const & parton){
       return parton.p.user_index() == EXTREMAL_JET_IDX;
     }
 
     template<class InputIterator>
     double FKL_ladder_weight(
         InputIterator begin_gluon, InputIterator end_gluon,
         CLHEP::HepLorentzVector const & q0,
         CLHEP::HepLorentzVector const & pa, CLHEP::HepLorentzVector const & pb,
         CLHEP::HepLorentzVector const & p1, CLHEP::HepLorentzVector const & pn,
         double lambda
     ){
       double wt = 1;
       auto qi = q0;
       for(auto gluon_it = begin_gluon; gluon_it != end_gluon; ++gluon_it){
         assert(gluon_it->type == pid::gluon);
         const auto g = to_HepLorentzVector(*gluon_it);
         const auto qip1 = qi - g;
 
         if(treat_as_extremal(*gluon_it)){
           wt *= C2Lipatovots(qip1, qi, pa, pb, lambda)*C_A;
         } else{
           wt *= C2Lipatovots(qip1, qi, pa, pb, p1, pn, lambda)*C_A;
         }
 
         qi = qip1;
       }
       return wt;
     }
 
     template<class InputIterator>
     std::vector <double> FKL_ladder_weight_mix(
         InputIterator begin_gluon, InputIterator end_gluon,
         CLHEP::HepLorentzVector const & q0_t, CLHEP::HepLorentzVector const & q0_b,
         CLHEP::HepLorentzVector const & pa, CLHEP::HepLorentzVector const & pb,
         CLHEP::HepLorentzVector const & p1, CLHEP::HepLorentzVector const & pn,
         const double lambda
     ){
       double wt_top = 1;
       double wt_bot = 1;
       double wt_mix = 1;
       auto qi_t = q0_t;
       auto qi_b = q0_b;
       for(auto gluon_it = begin_gluon; gluon_it != end_gluon; ++gluon_it){
         assert(gluon_it->type == pid::gluon);
         const auto g = to_HepLorentzVector(*gluon_it);
         const auto qip1_t = qi_t - g;
         const auto qip1_b = qi_b - g;
         if(treat_as_extremal(*gluon_it)){
           wt_top *= C2Lipatovots(qip1_t, qi_t, pa, pb, lambda)*C_A;
           wt_bot *= C2Lipatovots(qip1_b, qi_b, pa, pb, lambda)*C_A;
           wt_mix *= C2Lipatovots_Mix(qip1_t, qi_t, qip1_b, qi_b, pa, pb, lambda)*C_A;
         } else{
           wt_top *= C2Lipatovots(qip1_t, qi_t, pa, pb, p1, pn, lambda)*C_A;
           wt_bot *= C2Lipatovots(qip1_b, qi_b, pa, pb, p1, pn, lambda)*C_A;
           wt_mix *= C2Lipatovots_Mix(qip1_t, qi_t, qip1_b, qi_b, pa, pb, p1, pn, lambda)*C_A;
         }
         qi_t = qip1_t;
         qi_b = qip1_b;
       }
       return {wt_top, wt_bot, wt_mix};
     }
 
     std::vector<Particle> tag_extremal_jet_partons( Event const & ev ){
       auto out_partons = filter_partons(ev.outgoing());
       if(out_partons.size() == ev.jets().size()){
         // no additional emissions in extremal jets, don't need to tag anything
         for(auto & parton: out_partons){
           parton.p.set_user_index(NO_EXTREMAL_JET_IDX);
         }
         return out_partons;
       }
       auto const & jets = ev.jets();
       std::vector<fastjet::PseudoJet> extremal_jets;
       if(! is_backward_g_to_h(ev)) {
         auto most_backward = begin(jets);
         // skip jets caused by unordered emission or qqbar
         if(ev.type() == event_type::unob || ev.type() == event_type::qqbar_exb){
           assert(jets.size() >= 2);
           ++most_backward;
         }
         extremal_jets.emplace_back(*most_backward);
       }
       if(! is_forward_g_to_h(ev)) {
         auto most_forward = end(jets) - 1;
         if(ev.type() == event_type::unof || ev.type() == event_type::qqbar_exf){
           assert(jets.size() >= 2);
           --most_forward;
         }
         extremal_jets.emplace_back(*most_forward);
       }
       const auto extremal_jet_indices = ev.particle_jet_indices(
         extremal_jets
       );
       assert(extremal_jet_indices.size() == out_partons.size());
       for(std::size_t i = 0; i < out_partons.size(); ++i){
         assert(is_parton(out_partons[i]));
         const int idx = (extremal_jet_indices[i]>=0)?
           EXTREMAL_JET_IDX:
           NO_EXTREMAL_JET_IDX;
         out_partons[i].p.set_user_index(idx);
       }
       return out_partons;
     }
 
     double tree_kin_jets_qqbar_mid(
         ParticleID aptype, ParticleID bptype,
         CLHEP::HepLorentzVector const & pa, CLHEP::HepLorentzVector const & pb,
         std::vector<Particle> const & partons,
         double lambda
     ){
      CLHEP::HepLorentzVector pq;
      CLHEP::HepLorentzVector pqbar;
       const auto backmidquark = std::find_if(
           begin(partons)+1, end(partons)-1,
           [](Particle const & s){ return s.type != pid::gluon; }
       );
 
       assert(backmidquark!=end(partons)-1);
 
       if (is_quark(backmidquark->type)){
         pq = to_HepLorentzVector(*backmidquark);
         pqbar = to_HepLorentzVector(*(backmidquark+1));
       }
       else {
         pqbar = to_HepLorentzVector(*backmidquark);
         pq = to_HepLorentzVector(*(backmidquark+1));
       }
 
       auto p1 = to_HepLorentzVector(partons[0]);
       auto pn = to_HepLorentzVector(partons[partons.size() - 1]);
 
       auto q0 = pa - p1;
       // t-channel momentum after qqbar
       auto qqbart = q0;
 
       const auto begin_ladder = cbegin(partons) + 1;
       const auto end_ladder_1 = (backmidquark);
       const auto begin_ladder_2 = (backmidquark+2);
       const auto end_ladder = cend(partons) - 1;
       for(auto parton_it = begin_ladder; parton_it < begin_ladder_2; ++parton_it){
         qqbart -= to_HepLorentzVector(*parton_it);
       }
 
       const int nabove = std::distance(begin_ladder, backmidquark);
 
       std::vector<CLHEP::HepLorentzVector> partonsHLV;
       partonsHLV.reserve(partons.size());
       for (std::size_t i = 0; i != partons.size(); ++i) {
         partonsHLV.push_back(to_HepLorentzVector(partons[i]));
       }
 
       const double current_factor = ME_qqbar_mid_current(
           aptype, bptype, nabove, pa, pb,
           pq, pqbar, partonsHLV
       );
 
       const double ladder_factor = FKL_ladder_weight(
           begin_ladder, end_ladder_1,
           q0, pa, pb, p1, pn,
           lambda
       )*FKL_ladder_weight(
           begin_ladder_2, end_ladder,
           qqbart, pa, pb, p1, pn,
           lambda
         );
       return current_factor*ladder_factor;
     }
 
 
     template<class InIter, class partIter>
     double tree_kin_jets_qqbar(InIter BeginIn, InIter EndIn, partIter BeginPart,
                               partIter EndPart, double lambda){
       const auto pgin = to_HepLorentzVector(*BeginIn);
       const auto pb   = to_HepLorentzVector(*(EndIn-1));
       const auto p1   = to_HepLorentzVector(*BeginPart);
       const auto p2   = to_HepLorentzVector(*(BeginPart+1));
       const auto pn   = to_HepLorentzVector(*(EndPart-1));
 
       assert((BeginIn)->type==pid::gluon); // Incoming a must be gluon.
       const double current_factor = ME_qqbar_current(
         (EndIn-1)->type, pgin, p1, p2, pn, pb
         )/(4.*(N_C*N_C - 1.));
       const double ladder_factor = FKL_ladder_weight(
           (BeginPart+2), (EndPart-1),
           pgin-p1-p2, pgin, pb, p1, pn, lambda
           );
 
       return current_factor*ladder_factor;
     }
 
     template<class InIter, class partIter>
     double tree_kin_jets_uno(InIter BeginIn, InIter EndIn, partIter BeginPart,
                              partIter EndPart, double lambda
     ){
 
       const auto pa = to_HepLorentzVector(*BeginIn);
       const auto pb = to_HepLorentzVector(*(EndIn-1));
 
       const auto pg = to_HepLorentzVector(*BeginPart);
       const auto p1 = to_HepLorentzVector(*(BeginPart+1));
       const auto pn = to_HepLorentzVector(*(EndPart-1));
 
       const double current_factor = ME_uno_current(
         (BeginIn)->type, (EndIn-1)->type, pg, pn, pb, p1, pa
       )/(4.*(N_C*N_C - 1.));
       const double ladder_factor = FKL_ladder_weight(
           (BeginPart+2), (EndPart-1),
           pa-p1-pg, pa, pb, p1, pn, lambda
           );
 
       return current_factor*ladder_factor;
     }
   } // namespace
 
   double MatrixElement::tree_kin_jets(Event const & ev) const {
     auto const & incoming = ev.incoming();
     const auto partons = tag_extremal_jet_partons(ev);
 
     if (ev.type()==event_type::FKL){
       const auto pa = to_HepLorentzVector(incoming[0]);
       const auto pb = to_HepLorentzVector(incoming[1]);
 
       const auto p1 = to_HepLorentzVector(partons.front());
       const auto pn = to_HepLorentzVector(partons.back());
       return ME_current(
         incoming[0].type, incoming[1].type,
         pn, pb, p1, pa
         )/(4.*(N_C*N_C - 1.))*FKL_ladder_weight(
           begin(partons) + 1, end(partons) - 1,
           pa - p1, pa, pb, p1, pn,
           param_.regulator_lambda
         );
     }
     if (ev.type()==event_type::unordered_backward){
       return tree_kin_jets_uno(incoming.begin(), incoming.end(),
                                partons.begin(), partons.end(),
                                param_.regulator_lambda);
     }
     if (ev.type()==event_type::unordered_forward){
       return tree_kin_jets_uno(incoming.rbegin(), incoming.rend(),
                                partons.rbegin(), partons.rend(),
                                param_.regulator_lambda);
     }
     if (ev.type()==event_type::extremal_qqbar_backward){
       return tree_kin_jets_qqbar(incoming.begin(), incoming.end(),
                                  partons.begin(), partons.end(),
                                  param_.regulator_lambda);
     }
     if (ev.type()==event_type::extremal_qqbar_forward){
       return tree_kin_jets_qqbar(incoming.rbegin(), incoming.rend(),
                                  partons.rbegin(), partons.rend(),
                                  param_.regulator_lambda);
     }
     if (ev.type()==event_type::central_qqbar){
       return tree_kin_jets_qqbar_mid(incoming[0].type, incoming[1].type,
                                      to_HepLorentzVector(incoming[0]),
                                      to_HepLorentzVector(incoming[1]),
                                      partons, param_.regulator_lambda);
    }
     throw std::logic_error("Cannot reweight non-resummable processes in Pure Jets");
   }
 
   namespace {
     double tree_kin_W_FKL(
         ParticleID aptype, ParticleID bptype,
         CLHEP::HepLorentzVector const & pa, CLHEP::HepLorentzVector const & pb,
         std::vector<Particle> const & partons,
         CLHEP::HepLorentzVector const & plbar, CLHEP::HepLorentzVector const & pl,
         double lambda, ParticleProperties const & Wprop
     ){
       auto p1 = to_HepLorentzVector(partons[0]);
       auto pn = to_HepLorentzVector(partons[partons.size() - 1]);
 
       const auto begin_ladder = cbegin(partons) + 1;
       const auto end_ladder = cend(partons) - 1;
 
       bool wc = aptype==partons[0].type; //leg b emits w
       auto q0 = pa - p1;
       if(!wc)
         q0 -= pl + plbar;
 
       const double current_factor = ME_W_current(
           aptype, bptype, pn, pb,
           p1, pa, plbar, pl, wc, Wprop
       );
 
       const double ladder_factor = FKL_ladder_weight(
           begin_ladder, end_ladder,
           q0, pa, pb, p1, pn,
           lambda
       );
       return current_factor*ladder_factor;
     }
 
 
     template<class InIter, class partIter>
     double tree_kin_W_uno(InIter BeginIn, partIter BeginPart,
                           partIter EndPart,
                           const CLHEP::HepLorentzVector & plbar,
                           const CLHEP::HepLorentzVector & pl,
                           double lambda, ParticleProperties const & Wprop
     ){
       const auto pa = to_HepLorentzVector(*BeginIn);
       const auto pb = to_HepLorentzVector(*(BeginIn+1));
 
       const auto pg = to_HepLorentzVector(*BeginPart);
       const auto p1 = to_HepLorentzVector(*(BeginPart+1));
       const auto pn = to_HepLorentzVector(*(EndPart-1));
 
       bool wc = (BeginIn)->type==(BeginPart+1)->type; //leg b emits w
       auto q0 = pa - p1 - pg;
       if(!wc)
         q0 -= pl + plbar;
 
       const double current_factor = ME_W_uno_current(
           (BeginIn)->type, (BeginIn+1)->type, pn, pb,
           p1, pa, pg, plbar, pl, wc, Wprop
       );
 
       const double ladder_factor = FKL_ladder_weight(
           BeginPart+2, EndPart-1,
           q0, pa, pb, p1, pn,
           lambda
       );
       return current_factor*ladder_factor;
     }
 
     template<class InIter, class partIter>
     double tree_kin_W_qqbar(InIter BeginIn, partIter BeginPart,
                             partIter EndPart,
                             const CLHEP::HepLorentzVector & plbar,
                             const CLHEP::HepLorentzVector & pl,
                             double lambda, ParticleProperties const & Wprop
     ){
       const bool swap_qqbar=is_quark(*BeginPart);
       const auto pa = to_HepLorentzVector(*BeginIn);
       const auto pb = to_HepLorentzVector(*(BeginIn+1));
       const auto pq = to_HepLorentzVector(*(BeginPart+(swap_qqbar?0:1)));
       const auto pqbar = to_HepLorentzVector(*(BeginPart+(swap_qqbar?1:0)));
       const auto p1 = to_HepLorentzVector(*(BeginPart));
       const auto pn = to_HepLorentzVector(*(EndPart-1));
 
       const bool wc = (BeginIn+1)->type!=(EndPart-1)->type; //leg b emits w
       auto q0 = pa - pq - pqbar;
       if(!wc)
         q0 -= pl + plbar;
 
       const double current_factor = ME_W_qqbar_current(
         (BeginIn+1)->type, pa, pb,
         pq, pqbar, pn, plbar, pl, wc, Wprop
       );
 
       const double ladder_factor = FKL_ladder_weight(
           BeginPart+2, EndPart-1,
           q0, pa, pb, p1, pn,
           lambda
       );
       return current_factor*ladder_factor;
     }
 
     double tree_kin_W_qqbar_mid(
         ParticleID aptype, ParticleID bptype,
         CLHEP::HepLorentzVector const & pa,
         CLHEP::HepLorentzVector const & pb,
         std::vector<Particle> const & partons,
         CLHEP::HepLorentzVector const & plbar, CLHEP::HepLorentzVector const & pl,
         double lambda, ParticleProperties const & Wprop
     ){
      CLHEP::HepLorentzVector pq;
      CLHEP::HepLorentzVector pqbar;
       const auto backmidquark = std::find_if(
           begin(partons)+1, end(partons)-1,
           [](Particle const & s){ return s.type != pid::gluon; }
       );
 
       assert(backmidquark!=end(partons)-1);
 
       if (is_quark(backmidquark->type)){
         pq = to_HepLorentzVector(*backmidquark);
         pqbar = to_HepLorentzVector(*(backmidquark+1));
       }
       else {
         pqbar = to_HepLorentzVector(*backmidquark);
         pq = to_HepLorentzVector(*(backmidquark+1));
       }
 
       auto p1 = to_HepLorentzVector(partons.front());
       auto pn = to_HepLorentzVector(partons.back());
 
       auto q0 = pa - p1;
       // t-channel momentum after qqbar
       auto qqbart = q0;
 
       bool wqq = backmidquark->type != -(backmidquark+1)->type; // qqbar emit W
       bool wc = !wqq && (aptype==partons.front().type); //leg b emits w
       assert(!wqq || !wc);
       if(wqq){ // emission from qqbar
         qqbart -= pl + plbar;
       } else if(!wc) { // emission from leg a
         q0 -= pl + plbar;
         qqbart -= pl + plbar;
       }
 
       const auto begin_ladder = cbegin(partons) + 1;
       const auto end_ladder_1 = (backmidquark);
       const auto begin_ladder_2 = (backmidquark+2);
       const auto end_ladder = cend(partons) - 1;
       for(auto parton_it = begin_ladder; parton_it < begin_ladder_2; ++parton_it){
         qqbart -= to_HepLorentzVector(*parton_it);
       }
 
       const int nabove = std::distance(begin_ladder, backmidquark);
       const int nbelow = std::distance(begin_ladder_2, end_ladder);
 
       std::vector<CLHEP::HepLorentzVector> partonsHLV;
       partonsHLV.reserve(partons.size());
       for (std::size_t i = 0; i != partons.size(); ++i) {
         partonsHLV.push_back(to_HepLorentzVector(partons[i]));
       }
 
       const double current_factor = ME_W_qqbar_mid_current(
           aptype, bptype, nabove, nbelow, pa, pb,
           pq, pqbar, partonsHLV, plbar, pl, wqq, wc, Wprop
       );
 
       const double ladder_factor = FKL_ladder_weight(
           begin_ladder, end_ladder_1,
           q0, pa, pb, p1, pn,
           lambda
       )*FKL_ladder_weight(
           begin_ladder_2, end_ladder,
           qqbart, pa, pb, p1, pn,
           lambda
         );
       return current_factor*ladder_factor;
     }
   } // namespace
 
   double MatrixElement::tree_kin_W(Event const & ev) const {
     using namespace event_type;
     auto const & incoming(ev.incoming());
 
   #ifndef NDEBUG
     // assert that there is exactly one decay corresponding to the W
     assert(ev.decays().size() == 1);
     auto const & w_boson{
       std::find_if(ev.outgoing().cbegin(), ev.outgoing().cend(),
         [] (Particle const & p) -> bool {
           return std::abs(p.type) == ParticleID::Wp;
         }) };
     assert(w_boson != ev.outgoing().cend());
     assert( static_cast<long int>(ev.decays().cbegin()->first)
         == std::distance(ev.outgoing().cbegin(), w_boson) );
   #endif
 
     // find decay products of W
     auto const & decay{ ev.decays().cbegin()->second };
     assert(decay.size() == 2);
     assert( ( is_anylepton(decay.at(0)) && is_anyneutrino(decay.at(1)) )
         || ( is_anylepton(decay.at(1)) && is_anyneutrino(decay.at(0)) ) );
 
     // get lepton & neutrino
     CLHEP::HepLorentzVector plbar;
     CLHEP::HepLorentzVector pl;
     if (decay.at(0).type < 0){
       plbar = to_HepLorentzVector(decay.at(0));
       pl = to_HepLorentzVector(decay.at(1));
     }
     else{
       pl = to_HepLorentzVector(decay.at(0));
       plbar = to_HepLorentzVector(decay.at(1));
     }
 
     const auto pa = to_HepLorentzVector(incoming[0]);
     const auto pb = to_HepLorentzVector(incoming[1]);
 
     const auto partons = tag_extremal_jet_partons(ev);
 
     if(ev.type() == FKL){
       return tree_kin_W_FKL(incoming[0].type, incoming[1].type,
                             pa, pb, partons, plbar, pl,
                             param_.regulator_lambda,
                             param_.ew_parameters.Wprop());
     }
     if(ev.type() == unordered_backward){
       return tree_kin_W_uno(cbegin(incoming), cbegin(partons),
                             cend(partons), plbar, pl,
                             param_.regulator_lambda,
                             param_.ew_parameters.Wprop());
     }
     if(ev.type() == unordered_forward){
       return tree_kin_W_uno(crbegin(incoming), crbegin(partons),
                             crend(partons), plbar, pl,
                             param_.regulator_lambda,
                             param_.ew_parameters.Wprop());
     }
     if(ev.type() == extremal_qqbar_backward){
       return tree_kin_W_qqbar(cbegin(incoming), cbegin(partons),
                               cend(partons), plbar, pl,
                               param_.regulator_lambda,
                               param_.ew_parameters.Wprop());
     }
     if(ev.type() == extremal_qqbar_forward){
       return tree_kin_W_qqbar(crbegin(incoming), crbegin(partons),
                               crend(partons), plbar, pl,
                               param_.regulator_lambda,
                               param_.ew_parameters.Wprop());
     }
     assert(ev.type() == central_qqbar);
     return tree_kin_W_qqbar_mid(incoming[0].type, incoming[1].type,
                                 pa, pb, partons, plbar, pl,
                                 param_.regulator_lambda,
                                 param_.ew_parameters.Wprop());
   }
 
   namespace /* WW */ {
     std::vector <double> tree_kin_WW_FKL(
         ParticleID aptype, ParticleID bptype,
         CLHEP::HepLorentzVector const & pa, CLHEP::HepLorentzVector const & pb,
         std::vector<Particle> const & partons,
         CLHEP::HepLorentzVector const & pl1bar, CLHEP::HepLorentzVector const & pl1,
         CLHEP::HepLorentzVector const & pl2bar, CLHEP::HepLorentzVector const & pl2,
         double lambda, ParticleProperties const & Wprop
     ){
       assert(is_anyquark(aptype));
       assert(is_anyquark(bptype));
 
       auto p1 = to_HepLorentzVector(partons[0]);
       auto pn = to_HepLorentzVector(partons[partons.size() - 1]);
 
       const std::vector <double> current_factor = ME_WW_current(
           aptype, bptype, pn, pb, p1, pa,
           pl1bar, pl1, pl2bar, pl2,
           Wprop
       );
 
       auto const begin_ladder = cbegin(partons) + 1;
       auto const end_ladder = cend(partons) - 1;
 
       // pa -> W1 p1, pb -> W2 + pn
       const auto q0 = pa - p1 - (pl1 + pl1bar);
       // pa -> W2 p1, pb -> W1 + pn
       const auto q1 = pa - p1 - (pl2 + pl2bar);
 
       const std::vector <double> ladder_factor = FKL_ladder_weight_mix(
         begin_ladder, end_ladder,
         q0, q1, pa, pb, p1, pn,
         lambda
       );
 
       assert(current_factor.size() == 3);
       assert(current_factor.size() == ladder_factor.size());
       std::vector<double> result(current_factor.size());
       for(size_t i=0; i<current_factor.size(); ++i){
         result[i] = K_q*K_q/(4.*(N_C*N_C - 1.))
           *current_factor.at(i)
           *ladder_factor.at(i);
       }
 
       const double t1_top = q0.m2();
       const double t2_top = (pb-pn-pl2bar-pl2).m2();
 
       const double t1_bot = q1.m2();
       const double t2_bot = (pb-pn-pl1bar-pl1).m2();
 
       result[0] /= t1_top * t2_top;
       result[1] /= t1_bot * t2_bot;
       result[2] /= sqrt(t1_top * t2_top * t1_bot * t2_bot);
       return result;
     }
   } // namespace
 
   std::vector <double> MatrixElement::tree_kin_WW(Event const & ev) const {
     using namespace event_type;
     auto const & incoming(ev.incoming());
 
     auto const pa = to_HepLorentzVector(incoming[0]);
     auto const pb = to_HepLorentzVector(incoming[1]);
 
     auto const partons = tag_extremal_jet_partons(ev);
 
     // W1 & W2
     assert(ev.decays().size() == 2);
 
     std::vector<CLHEP::HepLorentzVector> plbar;
     std::vector<CLHEP::HepLorentzVector> pl;
 
     for(auto const & decay_pair : ev.decays()) {
       auto const decay = decay_pair.second;
       // TODO: how to label W1, W2
       if(decay.at(0).type < 0) {
         plbar.emplace_back(to_HepLorentzVector(decay.at(0)));
         pl   .emplace_back(to_HepLorentzVector(decay.at(1)));
       }
       else {
         pl   .emplace_back(to_HepLorentzVector(decay.at(0)));
         plbar.emplace_back(to_HepLorentzVector(decay.at(1)));
       }
     }
 
     if(ev.type() == FKL) {
 
       return tree_kin_WW_FKL(
           incoming[0].type, incoming[1].type,
           pa, pb, partons,
           plbar[0], pl[0], plbar[1], pl[1],
           param_.regulator_lambda,
           param_.ew_parameters.Wprop()
       );
 
     }
     throw std::logic_error("Can only reweight FKL events in WW");
   }
 
   namespace{
     std::vector <double> tree_kin_Z_FKL(
         const ParticleID aptype, const ParticleID bptype,
         CLHEP::HepLorentzVector const & pa, CLHEP::HepLorentzVector const & pb,
         std::vector<Particle> const & partons,
         CLHEP::HepLorentzVector const & plbar, CLHEP::HepLorentzVector const & pl,
         const double lambda, ParticleProperties const & Zprop,
         const double stw2, const double ctw
     ){
       const auto p1 = to_HepLorentzVector(partons[0]);
       const auto pn = to_HepLorentzVector(partons[partons.size() - 1]);
 
       const auto begin_ladder = cbegin(partons) + 1;
       const auto end_ladder = cend(partons) - 1;
 
       const std::vector <double> current_factor = ME_Z_current(
           aptype, bptype, pn, pb, p1, pa,
           plbar, pl, Zprop, stw2, ctw
       );
 
       std::vector <double> ladder_factor;
       if(is_gluon(bptype)){
         // This is a qg event
         const auto q0 = pa-p1-plbar-pl;
         ladder_factor.push_back(FKL_ladder_weight(begin_ladder, end_ladder,
                                                   q0, pa, pb, p1, pn, lambda));
       } else if(is_gluon(aptype)){
         // This is a gq event
         const auto q0 = pa-p1;
         ladder_factor.push_back(FKL_ladder_weight(begin_ladder, end_ladder,
                                                   q0, pa, pb, p1, pn, lambda));
       } else {
         // This is a qq event
         const auto q0 = pa-p1-plbar-pl;
         const auto q1 = pa-p1;
         ladder_factor=FKL_ladder_weight_mix(begin_ladder, end_ladder,
                                             q0, q1, pa, pb, p1, pn, lambda);
       }
 
       std::vector <double> result;
       for(size_t i=0; i<current_factor.size(); ++i){
         result.push_back(current_factor.at(i)*ladder_factor.at(i));
       }
       return result;
     }
 
     template<class InIter, class partIter>
     std::vector <double> tree_kin_Z_uno(InIter BeginIn, partIter BeginPart, partIter EndPart,
                                         const CLHEP::HepLorentzVector & plbar,
                                         const CLHEP::HepLorentzVector & pl,
                                         const double lambda, ParticleProperties const & Zprop,
                                         const double stw2, const double ctw){
       const auto pa = to_HepLorentzVector(*BeginIn);
       const auto pb = to_HepLorentzVector(*(BeginIn+1));
 
       const auto pg = to_HepLorentzVector(*BeginPart);
       const auto p1 = to_HepLorentzVector(*(BeginPart+1));
       const auto pn = to_HepLorentzVector(*(EndPart-1));
 
       const ParticleID aptype = (BeginIn)->type;
       const ParticleID bptype = (BeginIn+1)->type;
 
       const std::vector <double> current_factor = ME_Z_uno_current(
           aptype, bptype, pn, pb, p1, pa, pg,
           plbar, pl, Zprop, stw2, ctw
       );
 
       std::vector <double> ladder_factor;
       if(is_gluon(bptype)){
         // This is a qg event
         const auto q0 = pa-pg-p1-plbar-pl;
         ladder_factor.push_back(FKL_ladder_weight(BeginPart+2, EndPart-1,
                                                   q0, pa, pb, p1, pn, lambda));
       }else if(is_gluon(aptype)){
         // This is a gq event
         const auto q0 = pa-pg-p1;
         ladder_factor.push_back(FKL_ladder_weight(BeginPart+2, EndPart-1,
                                                   q0, pa, pb, p1, pn, lambda));
       }else{
         // This is a qq event
         const auto q0 = pa-pg-p1-plbar-pl;
         const auto q1 = pa-pg-p1;
         ladder_factor=FKL_ladder_weight_mix(BeginPart+2, EndPart-1,
                                             q0, q1, pa, pb, p1, pn, lambda);
       }
 
       std::vector <double> result;
       for(size_t i=0; i<current_factor.size(); ++i){
         result.push_back(current_factor.at(i)*ladder_factor.at(i));
       }
       return result;
     }
 
   } // namespace
 
   std::vector<double> MatrixElement::tree_kin_Z(Event const & ev) const {
     using namespace event_type;
     auto const & incoming(ev.incoming());
 
     // find decay products of Z
     auto const & decay{ ev.decays().cbegin()->second };
     assert(decay.size() == 2);
     assert(is_anylepton(decay.at(0)) && !is_anyneutrino(decay.at(0))
            && decay.at(0).type==-decay.at(1).type);
 
     // get leptons
     CLHEP::HepLorentzVector plbar;
     CLHEP::HepLorentzVector pl;
     if (decay.at(0).type < 0){
       plbar = to_HepLorentzVector(decay.at(0));
       pl = to_HepLorentzVector(decay.at(1));
     }
     else{
       pl = to_HepLorentzVector(decay.at(0));
       plbar = to_HepLorentzVector(decay.at(1));
     }
 
     const auto pa = to_HepLorentzVector(incoming[0]);
     const auto pb = to_HepLorentzVector(incoming[1]);
 
     const auto partons = tag_extremal_jet_partons(ev);
 
     const double stw2 = param_.ew_parameters.sin2_tw();
     const double ctw  = param_.ew_parameters.cos_tw();
 
     if(ev.type() == FKL){
       return tree_kin_Z_FKL(incoming[0].type, incoming[1].type,
                             pa, pb, partons, plbar, pl,
                             param_.regulator_lambda,
                             param_.ew_parameters.Zprop(),
                             stw2, ctw);
     }
     if(ev.type() == unordered_backward){
       return tree_kin_Z_uno(cbegin(incoming), cbegin(partons),
                             cend(partons), plbar, pl,
                             param_.regulator_lambda,
                             param_.ew_parameters.Zprop(),
                             stw2, ctw);
     }
     if(ev.type() == unordered_forward){
       return tree_kin_Z_uno(crbegin(incoming), crbegin(partons),
                             crend(partons), plbar, pl,
                             param_.regulator_lambda,
                             param_.ew_parameters.Zprop(),
                             stw2, ctw);
     }
     throw std::logic_error("Can only reweight FKL or uno processes in Z+Jets");
   }
 
   double MatrixElement::tree_kin_Higgs(Event const & ev) const {
     if(ev.outgoing().front().type == pid::Higgs){
       return tree_kin_Higgs_first(ev);
     }
     if(ev.outgoing().back().type == pid::Higgs){
       return tree_kin_Higgs_last(ev);
     }
     return tree_kin_Higgs_between(ev);
   }
 
   // kinetic matrix element square for backward Higgs emission
   // cf. eq:ME_h_jets_peripheral in developer manual,
   // but without factors \alpha_s and the FKL ladder
   double MatrixElement::MH2_backwardH(
     const ParticleID type_forward,
     CLHEP::HepLorentzVector const & pa,
     CLHEP::HepLorentzVector const & pb,
     CLHEP::HepLorentzVector const & pH,
     CLHEP::HepLorentzVector const & pn
   ) const {
     using namespace currents;
     const double vev = param_.ew_parameters.vev();
     return K(type_forward, pn, pb)*ME_H_gq(
       pH, pa, pn, pb,
       param_.Higgs_coupling.mt, param_.Higgs_coupling.include_bottom,
       param_.Higgs_coupling.mb, vev
     )/(4.*(N_C*N_C - 1)*(pa-pH).m2()*(pb-pn).m2());
   }
 
   // kinetic matrix element square for backward unordered emission
   // and forward g -> Higgs
   double MatrixElement::MH2_unob_forwardH(
     CLHEP::HepLorentzVector const & pa,
     CLHEP::HepLorentzVector const & pb,
     CLHEP::HepLorentzVector const & pg,
     CLHEP::HepLorentzVector const & p1,
     CLHEP::HepLorentzVector const & pH
   ) const {
     using namespace currents;
     const double vev = param_.ew_parameters.vev();
 
     constexpr double K_f1 = K_q;
 
     constexpr double nhel = 4.;
 
     return K_f1*ME_juno_jgH(
       pg, p1, pa, pH, pb,
       param_.Higgs_coupling.mt, param_.Higgs_coupling.include_bottom,
       param_.Higgs_coupling.mb, vev
     )/(nhel*(N_C*N_C - 1)*(pa - p1 - pg).m2()*(pb - pH).m2());
   }
 
   double MatrixElement::tree_kin_Higgs_first(Event const & ev) const {
     auto const & incoming = ev.incoming();
     auto const & outgoing = ev.outgoing();
     assert(outgoing.front().type == pid::Higgs);
 
     if(is_anyquark(incoming.front())) {
       assert(incoming.front().type == outgoing[1].type);
       return tree_kin_Higgs_between(ev);
     }
 
     const auto partons = tag_extremal_jet_partons(ev);
 
     const auto pa = to_HepLorentzVector(incoming.front());
     const auto pb = to_HepLorentzVector(incoming.back());
 
     const auto pH = to_HepLorentzVector(outgoing.front());
 
     const auto end_ladder = end(partons) - ((ev.type() == event_type::unof)?2:1);
     const auto pn = to_HepLorentzVector(*end_ladder);
 
     const double ladder = FKL_ladder_weight(
         begin(partons), end_ladder,
         pa - pH, pa, pb, pa, pn,
         param_.regulator_lambda
     );
 
     if(ev.type() == event_type::unof) {
       const auto pg = to_HepLorentzVector(outgoing.back());
       return MH2_unob_forwardH(
         pb, pa, pg, pn, pH
       )*ladder;
     }
 
     return MH2_backwardH(
       incoming.back().type,
       pa, pb, pH, pn
     )*ladder;
   }
 
   double MatrixElement::tree_kin_Higgs_last(Event const & ev) const {
     auto const & incoming = ev.incoming();
     auto const & outgoing = ev.outgoing();
     assert(outgoing.back().type == pid::Higgs);
 
     if(is_anyquark(incoming.back())) {
       assert(incoming.back().type == outgoing[outgoing.size()-2].type);
       return tree_kin_Higgs_between(ev);
     }
 
     const auto partons = tag_extremal_jet_partons(ev);
 
     const auto pa = to_HepLorentzVector(incoming.front());
     const auto pb = to_HepLorentzVector(incoming.back());
 
     const auto pH = to_HepLorentzVector(outgoing.back());
 
     auto begin_ladder = begin(partons) + 1;
     auto q0 = pa - to_HepLorentzVector(partons.front());
     if(ev.type() == event_type::unob) {
       q0 -= to_HepLorentzVector(*begin_ladder);
       ++begin_ladder;
     }
     const auto p1 = to_HepLorentzVector(*(begin_ladder - 1));
 
     const double ladder = FKL_ladder_weight(
         begin_ladder, end(partons),
         q0, pa, pb, p1, pb,
         param_.regulator_lambda
     );
 
     if(ev.type() == event_type::unob) {
       const auto pg = to_HepLorentzVector(outgoing.front());
       return MH2_unob_forwardH(
         pa, pb, pg, p1, pH
       )*ladder;
     }
 
     return MH2_backwardH(
       incoming.front().type,
       pb, pa, pH, p1
     )*ladder;
   }
 
   namespace {
     template<class InIter, class partIter>
     double tree_kin_Higgs_uno(InIter BeginIn, InIter EndIn, partIter BeginPart,
                               partIter EndPart,
                               CLHEP::HepLorentzVector const & qH,
                               CLHEP::HepLorentzVector const & qHp1,
                               double mt, bool inc_bot, double mb, double vev
     ){
 
       const auto pa = to_HepLorentzVector(*BeginIn);
       const auto pb = to_HepLorentzVector(*(EndIn-1));
 
       const auto pg = to_HepLorentzVector(*BeginPart);
       const auto p1 = to_HepLorentzVector(*(BeginPart+1));
       const auto pn = to_HepLorentzVector(*(EndPart-1));
 
       return ME_Higgs_current_uno(
         (BeginIn)->type, (EndIn-1)->type, pg, pn, pb, p1, pa,
         qH, qHp1, mt, inc_bot, mb, vev
         );
     }
   } // namespace
 
 
   double MatrixElement::tree_kin_Higgs_between(Event const & ev) const {
     using namespace event_type;
     auto const & incoming = ev.incoming();
     auto const & outgoing = ev.outgoing();
 
     const auto the_Higgs = std::find_if(
         begin(outgoing), end(outgoing),
         [](Particle const & s){ return s.type == pid::Higgs; }
     );
     assert(the_Higgs != end(outgoing));
     const auto pH = to_HepLorentzVector(*the_Higgs);
     const auto partons = tag_extremal_jet_partons(ev);
 
     const auto pa = to_HepLorentzVector(incoming[0]);
     const auto pb = to_HepLorentzVector(incoming[1]);
 
     auto p1 = to_HepLorentzVector(
         partons[(ev.type() == unob)?1:0]
     );
     auto pn = to_HepLorentzVector(
         partons[partons.size() - ((ev.type() == unof)?2:1)]
     );
 
     auto first_after_Higgs = begin(partons) + (the_Higgs-begin(outgoing));
     assert(
         (first_after_Higgs == end(partons) && (
             (ev.type() == unob)
             || partons.back().type != pid::gluon
         ))
         || first_after_Higgs->rapidity() >= the_Higgs->rapidity()
     );
     assert(
         (first_after_Higgs == begin(partons) && (
             (ev.type() == unof)
             || partons.front().type != pid::gluon
         ))
         || (first_after_Higgs-1)->rapidity() <= the_Higgs->rapidity()
     );
     // always treat the Higgs as if it were in between the extremal FKL partons
     if(first_after_Higgs == begin(partons)) ++first_after_Higgs;
     else if(first_after_Higgs == end(partons)) --first_after_Higgs;
 
     // t-channel momentum before Higgs
     auto qH = pa;
     for(auto parton_it = begin(partons); parton_it != first_after_Higgs; ++parton_it){
       qH -= to_HepLorentzVector(*parton_it);
     }
 
     auto q0 = pa - p1;
     auto begin_ladder = begin(partons) + 1;
     auto end_ladder = end(partons) - 1;
 
     double current_factor = NAN;
     if(ev.type() == FKL){
       current_factor = ME_Higgs_current(
           incoming[0].type, incoming[1].type,
           pn, pb, p1, pa, qH, qH - pH,
           param_.Higgs_coupling.mt,
           param_.Higgs_coupling.include_bottom, param_.Higgs_coupling.mb,
           param_.ew_parameters.vev()
       );
     }
     else if(ev.type() == unob){
       current_factor = tree_kin_Higgs_uno(
         begin(incoming), end(incoming), begin(partons),
         end(partons), qH, qH-pH, param_.Higgs_coupling.mt,
         param_.Higgs_coupling.include_bottom, param_.Higgs_coupling.mb,
         param_.ew_parameters.vev()
         );
       const auto p_unob = to_HepLorentzVector(partons.front());
       q0 -= p_unob;
       p1 += p_unob;
       ++begin_ladder;
     }
     else if(ev.type() == unof){
       current_factor = tree_kin_Higgs_uno(
         rbegin(incoming), rend(incoming), rbegin(partons),
         rend(partons), qH-pH, qH, param_.Higgs_coupling.mt,
         param_.Higgs_coupling.include_bottom, param_.Higgs_coupling.mb,
         param_.ew_parameters.vev()
         );
       pn += to_HepLorentzVector(partons.back());
       --end_ladder;
     }
     else{
       throw std::logic_error("Can only reweight FKL or uno processes in H+Jets");
     }
 
     const double ladder_factor = FKL_ladder_weight(
         begin_ladder, first_after_Higgs,
         q0, pa, pb, p1, pn,
         param_.regulator_lambda
     )*FKL_ladder_weight(
         first_after_Higgs, end_ladder,
         qH - pH, pa, pb, p1, pn,
         param_.regulator_lambda
     );
     return current_factor/(4.*(N_C*N_C-1.))*ladder_factor;
   }
 
   namespace {
     double get_AWZH_coupling(Event const & ev, double alpha_s, double alpha_w) {
       std::vector<Particle> bosons = filter_AWZH_bosons(ev.outgoing());
 
       if(bosons.empty()) {
         return 1.;
       }
 
       if(bosons.size() == 1) {
         switch(bosons[0].type){
         case pid::Higgs:
           return alpha_s*alpha_s;
         case pid::Wp:
         case pid::Wm:
           return alpha_w*alpha_w;
         case pid::Z_photon_mix:
           return alpha_w*alpha_w;
         // TODO
         case pid::photon:
         case pid::Z:
         default:
           throw not_implemented("Emission of boson of unsupported type");
         }
       }
 
       if(bosons.size() == 2) {
         if(bosons[0].type == pid::Wp && bosons[1].type == pid::Wp) {
           return alpha_w*alpha_w*alpha_w*alpha_w;
         }
         else if(bosons[0].type == pid::Wm && bosons[1].type == pid::Wm) {
           return alpha_w*alpha_w*alpha_w*alpha_w;
         }
 
         throw not_implemented("Emission of bosons of unsupported type");
       }
 
 
       throw not_implemented("Emission of >2 bosons is unsupported");
     }
   } // namespace
 
   double MatrixElement::tree_param(Event const & ev, double mur) const {
     assert(is_resummable(ev.type()));
 
     const auto begin_partons = ev.begin_partons();
     const auto end_partons = ev.end_partons();
     const auto num_partons = std::distance(begin_partons, end_partons);
     const double alpha_s = alpha_s_(mur);
     const double gs2 = 4.*M_PI*alpha_s;
     double res = std::pow(gs2, num_partons);
     if(param_.log_correction){
       // use alpha_s(q_perp), evolved to mur
       assert(num_partons >= 2);
       const auto first_emission = std::next(begin_partons);
       const auto last_emission = std::prev(end_partons);
       for(auto parton = first_emission; parton != last_emission; ++parton){
         res *= 1. + alpha_s/(2.*M_PI)*BETA0*std::log(mur/parton->perp());
       }
     }
     return get_AWZH_coupling(ev, alpha_s, param_.ew_parameters.alpha_w())*res;
   }
 
 } // namespace HEJ