CDECK  ID>, HWREXQ.

*CMZ :-        -02/06/94  11.02.47  by  Mike Seymour

*-- Author :    David Ward, modified by Bryan Webber and Mike Seymour

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

      FUNCTION HWREXQ(AV,XMAX)

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

C     Random number from dN/d(x**2)=EXQ(-b*x) with mean AV,

C     But truncated at XMAX

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

      DOUBLE PRECISION HWREXQ,HWR,AV,B,BXMAX,R1,R2,XMAX,R,RMIN

      EXTERNAL HWR

      B=2./AV

      BXMAX=B*XMAX

      IF (BXMAX.LT.50) THEN

        RMIN=EXP(-BXMAX)

      ELSE

        RMIN=0

      ENDIF

 10   R1=HWR()*(1-RMIN)+RMIN

      R2=HWR()*(1-RMIN)+RMIN

      R=R1*R2

      IF (R.LT.RMIN) GOTO 10

      HWREXQ=-LOG(R)/B

      END