[Rivet] Fwd: FourMomentum mass

[Hendrik Hoeth]

Thus spake Sercan Sen (Sercan.Sen at cern.ch):

> It seems that it's looking for mass2() > 0 to return mass value,
> however, it does not work, I guess.

That's just an assert, meaning that if you compile with debug enabled
the program will bail out if the mass is negative, and if you compile
without debugging then the assert line is not present at all.

-- 
One doesn't discover new lands without consenting to lose
sight of the shore for a very long time.    -- André Gide