c     =================================================
      double precision function fdisc(x,y)
c     =================================================
      implicit none

      double precision x,y,x0,y0,r0

      common/cdisc/ x0,y0,r0

c     # circle of radius r0:
      fdisc = (x-x0)**2 + (y-y0)**2 - r0**2

      return
      end
