1.
Q:  ALGPRE ERROR   : XXX is not a valid function.  when using 
   Call drift_mc_electron(0.1,0.0001,0,Vstatus,Vtime,Vmulti,Vatt)
   Call fill_histogram(Hstatus,{Vstatus})

A: most of case Vstatus is a string, occationally, it is numerical value.  I suspect here, the Vstatus is string so can't fill histogram.