[Rivet] Rivet 2.4 and xsec/event counter information

[Frank Siegert]

>>>> Yes. It would have to be mentioned in the specs for HepMC3 that the
>>>> event number should be interpreted as the number of events generated so
>>>> far, and that zero-weight events may be left out.
>>>
>>>
>>> Which would require a lot of care in usage like ATLAS' generator
>>> filters, where events in the HepMC are discarded before writing to disk
>>> based on kinematic (etc.) selections... compatibility with that needs to
>>> be considered, at least.
>>
>>
>> Won't that still work? Let's say you originally have events numbered
>>
>> E 1 ...
>> E 452 ...
>> E 1642 ...
>>
>> and you leave out the middle one, you still get the correct "number of
>> attempts" from the third event.
>
>
> Ah, I see. I was somehow thinking that the individual deltas between event
> numbers would be important.

But it still won't work with the existing ATLAS workflow, because it
would mean that in these cases you double-ignore the filtered events
when applying the discarded event efficiency on top of cross section *
filter efficiency.

Cheers,
Frank