tuned signature (again, amending f32ac01aef5e), e.g. relevant for Isabelle/DOF;
package Simple_Greatest_Common_Divisor
is
--# function Gcd (A, B : Natural) return Natural;
procedure G_C_D (M, N : in Natural; G : out Natural);
--# derives G from M, N;
--# pre N > 0;
--# post G = Gcd (M, N);
end Simple_Greatest_Common_Divisor;