[Rivet] HepMV version issue

[Hendrik Hoeth]

Hi James,

without knowing what you are doing, it is fairly difficult to debug
anything. It seems you are getting/producing particles for which the
four-momentum yields a negative mass. So I'd try and add printouts to
see what's going on.

Cheers,

    Hendrik

-- 
Man is the best computer we can put aboard a spacecraft, and
the only one that can be mass produced with unskilled labor.
                                        -- Wernher von Braun