Tagged types (beginner's question) !

[Vincent DIEMUNSCH <"vincent.diemunsch"@edf.fr@NO.SPAM.PLEASE>]

Hello,

I have a simple question concerning tagged types. Suppose that we have :

    type Rationnel is abstract new Ada.Finalization.Controlled with null
record;

    type Fraction is new Rationnel with record
          Num�rateur    : integer;
          D�nominateur : positive;
       end record;

    type Entier is new Fraction with null record;

First Question :
    Can I override the primitive operation "Adjust" of the controlled
type with the following :
        procedure Adjust (F : in out Fraction'Class);
    instead of :
        procedure Adjust( F : in out Fraction);

Second question :
    How can Adjust (F : in out Fraction'Class) return an object of type
Entier instead of Fraction ?
    I tried something like
        F := E; --(with E : Entier)
    and many other more complicated things but I always have the same
error message from GNAT :
        "Dynamiclly tagged expression required".

Looking in the ARM and the user guide from GNAT, I could not get what
this message meant !
Any help would be greatly appreciated. Thanks in advance.

Vincent DIEMUNSCH