[Rivet] A comment on InvMassFinalState

[Adam Davison]

Hi,

I've been trying to use the InvMassFinalState class but the interface 
seems a bit difficult. It returns all particles that could be used to 
make, for example a Z, excluding duplicates. This means that if there's 
more than one candidate in an event, it can do some wierd things, like 
return a list of 3 or 5 particles. This makes it quite hard to apply any 
further cuts to the candidates, like pt, since in order to identify 
which particles are actually matching pairs, you have to loop over 
everything again anyway.

The ZFinder, which is the only place in Rivet I saw this being used 
returns if the size of the list isn't 2. Which unless I've 
misunderstood, potentially throws away events with good Z candidates in.

Adam