FUNCTION poidev(xm,idum)
      INTEGER idum
      REAL poidev,xm,PI
      PARAMETER (PI=3.141592654)
CU    USES gammln,ran1
      REAL alxm,em,g,oldm,sq,t,y,gammln,ran1
      SAVE alxm,g,oldm,sq
      DATA oldm /-1./
      if (xm.lt.12.)then
        if (xm.ne.oldm) then
          oldm=xm
          g=exp(-xm)
        endif
        em=-1
        t=1.
2       em=em+1.
        t=t*ran1(idum)
        if (t.gt.g) goto 2
      else
        if (xm.ne.oldm) then
          oldm=xm
          sq=sqrt(2.*xm)
          alxm=log(xm)
          g=xm*alxm-gammln(xm+1.)
        endif
1       y=tan(PI*ran1(idum))
        em=sq*y+xm
        if (em.lt.0.) goto 1
        em=int(em)
        t=0.9*(1.+y**2)*exp(em*alxm-gammln(em+1.)-g)
        if (ran1(idum).gt.t) goto 1
      endif
      poidev=em
      return
      END
C  (C) Copr. 1986-92 Numerical Recipes Software .