algorithm gcd(a,n)

      input: postive integers a, n  with a <= n.
     output: greatest common divisor of a and n.

  begin

     g0 = a;

     g1 = n;

     while ( g0 <> 0) do
  
        begin

           g = g1 mod g0;

           g1 = g0;

           g0 = g;

        end

      gcd = g1;

  end