What makes a procedure call 'dispatching' in Ada?

[Markus Schoepflin <nospam@no.spam>]

Hello,

I'm trying to fell may way around object oriented Ada programming, and I 
think I must be missing something absolutely basic. Please consider the 
following package:

---%<---
package FOOS is

    type FOO is abstract tagged null record;

    procedure P (THIS : in FOO);
    procedure A (THIS : in FOO) is abstract;

end FOOS;

package body FOOS is

    procedure P (THIS : in FOO)
    is
    begin
       A (THIS);
    end;

end FOOS;
--->%---

When trying to compile this, I get:

foos.adb:6:07: call to abstract function must be dispatching
gnatmake: "foos.adb" compilation error

What is the compiler trying to tell me here? And how do I go about calling 
abstract procedures?

TIA,
Markus