function gami (a, x)
  c july 1977 edition.  w. fullerton, c3, los alamos scientific lab.
  c
  c evaluate the incomplete gamma function defined by
  c
  c gami = integral from t = 0 to x of exp(-t) * t**(a-1.0) .
  c
  c gami is evaluated for positive values of a and non-negative values
  c of x.  a slight deterioration of 2 or 3 digits accuracy will occur
  c when gami is very large or very small, because logarithmic variables
  c are used.
  c
        external alngam, gamit
  c
        if (a.le.0.0) call seteru (25hgami    a must be gt zero, 25, 1, 2)
        if (x.lt.0.0) call seteru (25hgami    x must be ge zero, 25, 2, 2)
  c
        gami = 0.0
        if (x.eq.0.0) return
  c
  c the only error possible in the expression below is a fatal overflow.
        factor = exp (alngam(a) + a*alog(x) )
  c
        gami = factor * gamit(a, x)
  c
        return
        end