There are some problems with the B to 4 mu and gamma mu mu decay models, which were originally from the EvtGenExtras package in LHCb:

1. The lepton spin loop iterators for the B0 modes in EvtbsToLLLLAmp and EvtbsToLLLHyperCPAmp need correcting to avoid zero amplitudes. See https://gitlab.cern.ch/evtgen/evtgen/-/commit/6838e2c7

2. The BQTOLLLLHYPERCP model regularly returns very high probability values, which constantly fail accept/reject.

3. The BSTOGLLISRFSR model is also regularly failing accept/reject.