c  $Header: /usr/people/flittner/radtran/tomrad/pv2.2/src/RCS/geopro.f,v 2.22 2000/08/30 16:38:09 flittner Exp $
      subroutine geopro(x)
c
c**********************************************************************
cccc
c     subroutine geopro
c
c     version aug 22,1977
c
c     purpose
c
c        compute extrapolated value of argument
c
c     method
c
c        using last three iterated values of argument, a geometric
c        series approx. is used to compute extrapolated terms.
c
c     calling sequence
c
c        call geopro(x)
c
c        variable    type    i/o   description
c        --------    ----    ---   -----------
c
c        x(4)        r*8     i/o   argument to be extrapolated
c                                  x(1),x(2),x(3) contain last three
c                                  iteration values of argument to be
c                                  extrapolated. extrapolated term is
c                                  stored in x(4)
c
c     external references
c        none
c
c     common areas referenced
c        none
c
c     analysis and programming
c        k.f. klenk p.m. smith sasc, aug 22 1977
c
c     modifications (date name purpose)
c
c     last modified 11/18/92 bye zia ahmad
c     fixed the overflow problem
c
cccc
c**********************************************************************
c
      implicit none
c passed
      real*8 x(4)
c local
      real*8 a,b,c
c
      a=x(3)-x(2)
      b=x(2)-x(1)
      c = dabs(b-a)
      if (c .gt. 1.0d-9) then
         x(4) = x(2) + a*b/(b-a)
      else
         x(4) = x(3)
      endif
      return
      end