private classes

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

Hi,

sorry for so many question, but always when I thought I understand it, I 
read further and see a new problem... so given the following source-code:
package Classes is
 type Base_1 is tagged private;

 type Derived_1 is new Base_1 with private;

 procedure Proc(B: Base_1);

 private
  type Base_1 is tagged record
   Number: Integer;
  end record;

  type Derived_1 is new Base_1 with null record;
end Classes;

package body Classes is
 procedure Proc(b: Base_1) is
 begin
  null;
 end Proc;
end Classes;

with Classes;

procedure main is
 D: Classes.Derived_1;
begin
 Classes.Proc(D);
end Main;

I get an error in calling Classes.Proc(D). Although I thought that cause 
Derived_1 inherits from Base_1 the call should be possible. In fact exactly 
this I found in the book from John English in chapter 14.5. Why can't I 
compile such, although I thought the function Proc is visible for clients of 
the package Classes.
My second question is then, if the above does not work (and I really wonder 
why) then, what would it make for a difference if instead of writing:
type Derived_1 is new Base_1 with private;
I write:
type Derived_1 is private;
and then in the private-part of the package:
type Derived_1 is new Base_1 with null_record;
What would be the difference between that and the definition above?

Thanks tons in advance, cause I really start to become desperate ;(.

Bye,
Rick