tuned signature (again, amending f32ac01aef5e), e.g. relevant for Isabelle/DOF;
package body Simple_Greatest_Common_Divisor
is
procedure G_C_D (M, N : in Natural; G : out Natural)
is
C, D, R : Natural;
begin
C := M; D := N;
while D /= 0
--# assert Gcd (C, D) = Gcd (M, N);
loop
R := C mod D;
C := D; D := R;
end loop;
G := C;
end G_C_D;
end Simple_Greatest_Common_Divisor;