Help in OOP - abstract, tagged and generic

Help in OOP - abstract, tagged and generic

[chico]

I would Like to write an abstract package that will implement Sets in
two ways, one with list and one with array, I need to declare a base
type and then derive from it. doe's anybody has something like it
or can help me write it.(I don't know how to write the spec for the
the derived type)

the base is :
with Gnat.IO; use Gnat.IO;

package Sets is 

generic
   type Set_Element is private;

package Abstract_Sets is
  
   type Set is abstract tagged private;

   function Empty return Set is abstract;
   --     Empty Set
   function Unit (Element: Set_Elemet) return Set is abstract;
   --     build Set with 1 element
   function Union (Left, Right: Set) return Set is abstract;
   --     union of two sets
   function intersection (Left, Right: Set) return Set is abstract;
   --     intersection of two sets
   procedure Take (From: in out Set;
                           Element: out Set_Elemet) is abstract;
   --     remove an element from a set

private
   type Set is abstract tagged null record;
end Abstract_Sets;