Interesting containers problem.

[Shark8 <onewingedshark@gmail.com>]

Ok, so let's say we have a type for Identifiers:

   SubType Identifier is String
   with Dynamic_Predicate => Is_Valid( Identifier )
     or else raise Parse_Error with "Invalid identifier: """ & Identifier & '"';
--...
   -- ACH is a rename for Ada.Characters.Handling.
   Function Is_Valid( Input: String ) return Boolean is
     ( Input'Length in Positive and then
       ACH.Is_Letter(Input(Input'First)) and then
       ACH.Is_Alphanumeric(Input(Input'Last)) and then
       (for all Ch of Input => Ch = '_' or ACH.Is_Alphanumeric(Ch)) and then
       (for all Index in Input'First..Positive'Pred(Input'Last) =>
          (if Input(Index) = '_' then Input(Index+1) /= '_')
       )
     );

And a few types for types:
   Type Data_Type is ( Integer, Float, Fixed );
   Type Variable_Data( Type_Value : Data_Type ) is
    case Type_Value is
     when Integer => Integer_Value : Standard.Integer;
     when Float   => Float_Value   : Standard.Float;
     when Fixed   => Fixed_Value   : Fixed_Type; -- Defined elsewhere.
    end case;
   end record;

Now we can define a Symbol-table using the standard containers simply with this:

  Package Symbol_Table is
    new Ada.Containers.Indefinite_Ordered_Maps(
        Key_Type     => Identifier,
        Element_Type => Variable_Data,
        "<"          => Ada.Strings.Less_Case_Insensitive,
        "="          => "="
    );

And that's all well and good; however, there is one limitation here that is revealed when you need nesting scopes -- there is, apparently, no way to define an Element_Type to feed to Indefinite_Ordered_Maps which is an instance of Indefinite_Ordered_Maps.Maps or an instance of Variable_Data (by way of a variant record; even one which is forward declared).

What would be the proper way to handle this sort of situation?