function randgs (xmean, sd)
  c
  c generate a normally distributed random number, i.e., generate random
  c numbers with a gaussian distribution.  these random numbers are not
  c exceptionally good -- especially in the tails of the distribution,
  c but this implementation is simple and suitable for most applications.
  c see r. w. hamming, numerical methods for scientists and engineers,
  c mcgraw-hill, 1962, pages 34 and 389.
  c
  c             input arguments --
  c xmean  the mean of the gaussian distribution.
  c sd     the standard deviation of the gaussian function
  c          exp (-1/2 * (x-xmean)**2 / sd**2)
  c
        external rand
  c
        randgs = -6.
        do 10 i=1,12
          randgs = randgs + rand(0.)
   10   continue
  c
        randgs = xmean + sd*randgs
  c
        return
        end