Semantic of the mod operator in Ada 83

[sphinx.moro@wanadoo.fr (SPHINX)]

I just wander what is the semantic of
What is the exact semantic
of the function mod  in that case?
-- Compiler is gnat 3.15
-- modular types are not used, because the mode shall be Ada83

with text_io;
use  text_io;
procedure counter is
  	Size : constant := 32;
  	type Counter is range 0 .. (2 ** 31) - 1;  -- = 0..2147483647
  	for  Counter'Size use Size;

	V1  : Counter := Counter'LAST;
	V2  : Counter := Counter'LAST;
	R   : Counter := 0;
	Z   : Counter := 0;
	function Add (Val1 :in Counter;Val2 :in Counter) return Counter is
	begin
	  return ((Val1+Val2) mod Counter'Last);
	end Add;
	package counter_io is new integer_io(Counter);use counter_io;
begin
	put("V1=");	counter_io.put(V1);new_line;
	put("V2=");	counter_io.put(V2);new_line;
	R := Add (V1, V2);
	put("R=");counter_io.put(R);new_line;
	--Result for GNAT 3.15 => R=-2
	put("LAST=");put(Counter'Last);new_line;
	R := V1+V2;
	put("R=");counter_io.put(R);new_line;
	Z := R;
	put("Z=");counter_io.put(Z);new_line;
end counter;

And why there is no containt error on the direct additions
Thanks for the help