using an Ada.Container as a private type

Hi,
    I'm writing a little financial application. I want to model a
stream of payments made at arbitrary periods (the periods are positive
numbers; they might be actual dates later on).

I think I want to declare this:


type Payment_Stream is private;

procedure Add_To_Payment_Stream(
            stream : in out Payment_Stream;
            period :  Positive;
            amount : Money );

.......

and then privately declare the payment stream as an Ordered Map, like:

private

   package Payment_Stream_Package is new Ada.Containers.Ordered_Maps(
      Element_Type => Money,
      Key_Type => Positive );

   type Payment_Stream is new Payment_Stream_Package.Map;

but I can't do this. Gnat complains that "type derived from tagged
type must have extension".
I'm fairly new to Ada but thought I was getting on top of it! What is
the principal that stops me doing this? What should I do instead?

thanks,

Graham Stark