implicit cast and inheritance

["Rick Santa-Cruz" <rick_santa_cruz75@msn.com>]

Hi,

I have a small question. See the following source-code:
package Pack is
    type Base is tagged
        record
            Value: Integer;
        end record;
    procedure Base_Proc(B: Base);

    type Derived is new Base with
        null record;
end Pack;

And now I have the following source for the main-procedure:
with Pack;
use Pack;

procedure main is
    B: Base;
    D: Derived;
    procedure Proc(B: Base) is
    begin
        null;
    end Proc;
begin
    Pack.Base_Proc(B);    -- 1
    Pack.Base_Proc(D);    -- 2
    Proc(B);   -- 3
    Proc(D);    -- 4
end main;

It is clear to me, that the (4) won't compile, but I can solve it in making 
a cast or defining Proc with 'Class. My question is now why I can call 
Base_Proc(D). It is clear to me that the class Derived inherit the 
Base_Proc-Procedure from its base-class, but what is with the parameter? Why 
does this work?
I have a suggestion why this works... cause it is in the same package... but 
how does it internly work? If it is in the same package, is then always 
implicitly done a cast, or is the procedure Base_Proc in the derived class 
"Derived" automatically defined with a parameter Type "Derived"?

Thanks a lot in advance,
Rick