Extending A Generic Signature Package

[dgibson@thalamus.cis.ohio-state.edu (david scott gibson)]

Hi.  Is any way in Ada to extend a generic signature package?  For
example, if I had the signature package Base_Interface:

generic

    type T is limited private;

    with procedure Op1(x: in out T);

package Base_Interface is end;

I would like to extend it to get a new interface with Op1 inherited.
The following code is illegal, but gives an idea of what I'd like to
do.

with Base_Interface;

generic

    type T is new Base_Interface.T;  -- ILLEGAL!!

    with procedure Op2(x: in out T);

package Extended_Interface is end;

I'd be particularly interested if there were some way of doing this
without using tagged types.  For the language lawyers, what would be
the technical barriers to allowing an extension of this sort (not
including there are other ways in Ada to achieve a similar result :-)?

Dave
dgibson@cis.ohio-state.edu