text: c     ============================
        subroutine dt4(v1, v2, ans)
  c     ============================
  c Multiply two 4-vectors or the form (E - px - py - pz)
  c
        implicit none
   
        real v1(4), v2(4)
        real ans
  
        ans =((v1(1)*v2(1))-(v1(2)*v2(2))-(v1(3)*v2(3))-(v1(4)*v2(4)))
   
        return
        end
  