Another problem with "interface"

[Robert_Matthews <ignored@ramatthews.free-online.co.uk>]

In using interface types with GNAT I have encountered another problem.
Consider the following package:

package Test is

   type A_Type is limited interface;

   procedure P (A : in out A_Type; D : Integer) is abstract;

   protected type New_A_Type is new A_Type
   with
      procedure P (D : Integer);
      --  other subprograms...
   private
      F : Integer;
   end New_A_Type;

   function Set_A return New_A_Type;

end Test;

GNAT gives an error for the function Set_A: 
"operation can be dispatching in only one type",
which leaves me mystified. Am I making another
dumb mistake? Please help!

The version of GNAT is GNAT GPL 2008 (20080521).

Thanks in advance,

Robert