REAL FUNCTION tfit(x)!   or FUNC(X,Y) for a 2-Dim histogram
         implicit none
c
         COMMON/PAWPAR/PAR(2)
         real b,m,RI,x,par
         vector m
c         m=.139
c         m=.938
c         print *,' mass=',m(1)
         b=x/sqrt(x*x+m(1)*m(1))
         RI=3375./1500.
         RI=1.
         tfit=RI*sqrt(par(1)**2+(par(2)/x/b)**2)
         END