CDECK  ID>, HWUPCM.

*CMZ :-        -26/04/91  11.11.56  by  Bryan Webber

*-- Author :    Bryan Webber

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

      FUNCTION HWUPCM(EM0,EM1,EM2)

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

C     C.M. MOMENTUM FOR DECAY MASSES EM0 -> EM1 + EM2

C     SET TO -1 BELOW THRESHOLD

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

      DOUBLE PRECISION HWUPCM,EM0,EM1,EM2,EMS,EMD

      EMS=ABS(EM1+EM2)

      EMD=ABS(EM1-EM2)

      IF (EM0.LT.EMS.OR.EM0.LT.EMD) THEN

        HWUPCM=-1.

      ELSEIF (EM0.EQ.EMS.OR.EM0.EQ.EMD) THEN

        HWUPCM=0.

      ELSE

        HWUPCM=SQRT((EM0+EMD)*(EM0-EMD)*

     &              (EM0+EMS)*(EM0-EMS))*.5/EM0

      ENDIF

      END