subroutine stdfixmass

C... force particles to lie on the mass shell
C    assume that phep(5) is the correct mass
C    ignore particles which have already decayed

      IMPLICIT NONE

#include "stdhep.inc"
#include "stdlun.inc"

      integer i
      double precision msq,p4,newp,oldp,newroot,ms

      do i=1,nhep
        if( isthep(i) .eq. 1 ) then
          msq = phep(4,i)**2-phep(1,i)**2-phep(2,i)**2-phep(3,i)**2
          p4 = phep(4,i)**2+phep(5,i)**2-msq
          if(p4.lt.0.d0)then
            p4 = phep(5,i)**2+phep(1,i)**2+phep(2,i)**2+phep(3,i)**2
          endif
          phep(4,i) = sqrt(p4)
        endif
      enddo
      return
      end