Is it possible to make a possibly self-referential type using containers?

[Shark8 <onewingedshark@gmail.com>]

Consider something like Forth, IDL, and ASN.1 where there recursion plays a big part in the underlying ideas: the former in its definition of "Word"*, and in the latter two in the defining of types**.

So, given that the most straightforward way to implement these would be in a self-referencing type it would be nice to be able to use containers for this, perhaps like:

Type Executable_Code(<>) is private; -- The type in which we store executable code.
Type Word(<>);

Package Word_List is new Ada.Containers.Indefinite_Vectors
  (Element_Type => Word, Index_Type => Positive);

Type Word( Is_List ) is record
  case Is_List is
    when True  => List : Word_List.Vector;
    when False => Item : Executable_Code;
end record;

The above would be nicely elegant insofar as maintenance goes as it's simple and straightforward. However, we cannot use that because Element_Type cannot be a incomplete-type, but it needs to be because the full type declaration requires the instantiation of the container. -- It also seems that Limited/private with clauses cannot help. 

* Word: a chunk of code to execute or a list of words [to execute].
** Something like SEQUENCE OF INTEGER is fundamental in IDL and ASN.1.