CDECK  ID>, HWULF4.

*CMZ :-        -05/11/95  19.33.42  by  Mike Seymour

*-- Author :    Adapted by Bryan Webber

C-----------------------------------------------------------------------

      SUBROUTINE HWULF4(PS,PI,PF)

C-----------------------------------------------------------------------

C     TRANSFORMS PI (GIVEN IN LAB) INTO PF (IN REST FRAME OF PS)

C     N.B. P(1,2,3,4) = (PX,PY,PZ,E); PS(5)=M

C-----------------------------------------------------------------------

      DOUBLE PRECISION PF4,FN,PS(5),PI(4),PF(4)

      IF (PS(4).EQ.PS(5)) THEN

        PF(1)= PI(1)

        PF(2)= PI(2)

        PF(3)= PI(3)

        PF(4)= PI(4)

      ELSE

        PF4  = (PI(4)*PS(4)-PI(3)*PS(3)

     &         -PI(2)*PS(2)-PI(1)*PS(1))/PS(5)

        FN   = (PF4+PI(4)) / (PS(4)+PS(5))

        PF(1)= PI(1) - FN*PS(1)

        PF(2)= PI(2) - FN*PS(2)

        PF(3)= PI(3) - FN*PS(3)

        PF(4)= PF4

      END IF

      END