From the Forum:
https://twist.phys.ualberta.ca/forum/index.php?site=twist&bn=twist_montecarlo&action=view&key=1074552211

Posted by David Gill (drgill@triumf.ca) , Monday Jan 19, 2004 15:39

I have just committed guswim.F. It is a routine by PeterG that chooses
the real field(from map) for beam particles(and their secondaries)
while the decay particles(and their secondaries) are tracked in a
uniform field. I also committed the routine Uniform_field_Makefile
that has guswim.o in the objects list. Only use this Makefile if you
want to run GEANT for this very unusual situation.