* - 4 = \f$w_4\f$, the two-photon system's invariant mass
* - 5 = xx6 = \f$\frac{1}{2}\left(1-\cos\theta^{\rm CM}_6\right)\f$ definition (3D rotation of the first outgoing lepton with respect to the two-photon centre-of-mass system). If the \a nm_ optimisation flag is set this angle coefficient value becomes
* with \f$a_{\rm map}=\frac{1}{2}\left(w_4-t_1-t_2\right)\f$, \f$b_{\rm map}=\frac{1}{2}\sqrt{\left(\left(w_4-t_1-t_2\right)^2-4t_1t_2\right)\left(1-4\frac{w_6}{w_4}\right)}\f$, and \f$\beta=\left(\frac{a_{\rm map}+b_{\rm map}}{a_{\rm map}-b_{\rm map}}\right)^{2x_5-1}\f$
* and the Jacobian element is scaled by a factor \f$\frac{1}{2}\frac{\left(a_{\rm map}^2-b_{\rm map}^2\cos^2\theta^{\rm CM}_6\right)}{a_{\rm map}b_{\rm map}}\log\left(\frac{a_{\rm map}+b_{\rm map}}{a_{\rm map}-b_{\rm map}}\right)\f$
* - 6 = _phicm6_, or \f$\phi_6^{\rm CM}\f$ the rotation angle of the dilepton system in the centre-of-mass
* system
* - 7 = \f$x_q\f$, \f$w_X\f$ mappings, as used in the single- and double-dissociative
* cases only
* \brief Compute the matrix element for a CE \f$\gamma\gamma\to\ell^{+}\ell^{-}\f$
* process
*/
class GamGamLL : public GenericProcess
{
public:
/// \brief Class constructor: set the mandatory parameters before integration and events generation
/// \param[in] params General process parameters (nopt = Optimisation, legacy from LPAIR)