Comment 3 for bug 2051256

Hi Olivier! I think that Py8 interface code is out of date now. HepMC's weight-ordering issue was fixed some time ago, so it can be correctly structured without being forced into alphabetical ordering: this code seems to be working round an issue that no longer exists. And of course it pre-dates the weight standard.

So the simplest thing would just be to update this interface to set the weights to names like "IRREG_scomp=...". That should be sufficient to get it working, but if the ordering could also be revisited -- now that that's possible. I note this whole thing is inside a "#ifdef HEPMC2HACK" preprocessor block, which is maybe only a workaround for specific HepMC versions, or at least is deactivated for HepMC3, so we should make sure that whatever fix is applied also works when you migrate to running to HepMC3 internally (which I think is not yet the case, right?)

In general it would obviously be best for as much of this Py8-specific logic as possible to live in the Pythia8 codebase as library functions, and hence will evolve and update "automatically" with Pythia features. We have a similar issue with the ATLAS Py8 interface. So I'll mention this also, next time we raise the idea of pushing most of the interfacing physics-logic into libPythia.