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