--  Allocator interface
generic
   type Context is private;
   type Element_Range is range <>;
   with Procedure Initialize (Ctx  : out Context:
                              Size :     Element_Range);
   with Procedure Allocate (Ctx    :     Context;
                            Length :     Element_Range;
                            Start  : out Element_Range);
   with Procedure Free (Ctx   : Context;
                        Start : Element_Range);
package Allocator_Generic is
end Allocator_Generic;

--  Allocator implementation
package Simple_Allocator is

   type Element_Range is Integer range 0 .. Integer'Last;
   type Context is record
      Size : Element_Range;
      ...
   end record;

   procedure Initialize (Ctx  : out Context
                         Size :     Element_Range);

   procedure Allocate (R :     Element_Range;
                       S : out Element_Range;
                       L : out Element_Range);

   procedure Free (R : Element_Range;
                   S : Element_Range);

   package Allocator is new Allocator_Generic (Element_Range, Initialize, Allocate, Free);

end Simple_Allocator;

--  Package using an arbitrary allocator
generic
   with package Allocator_Generic (<>);
package Block_Allocator is
end Block_Allocator;