Glueing using Generic Units

[Fingertip <kammar@deletethispart.actcom.co.il>]

Hey everyone!

I'm interested in creating a generic package that is really flexible. What 
I mean is that instead of importing another package using a 'with' clause, 
which binds me to a certain interface of the 'with'ed unit, I want to glue 
the units together with a generic interface.

(As far as I know, this is a known and acceptable technique)

For example, my package requires a hash table and search tree, so I create 
these two interface packages:

generic
  type hash_item_type is private;
  with procedure add...
  with procedure find...
  -- et cetra
package hash_interface is
end hash_interface;

<the same thing goes for tree_interface>

Now I can create a generic implementaion of my package, independant of any 
special implementation of the two data structures:

generic
  with package Hash is new hash_interface (<>);
  with package Tree is new tree_interface (<>);
package My_Package is
  -- <Here comes my spec>
end My_Package;

Now comes my question:
In this definition set, the two generic package parameters are disjoint. 
That is, I gave no restriction on their parameters.

However, sometimes I'd want to restrict the generic parameters a little 
bit, for example, demand that Hash and Tree will store the same data type.

This kind of thing should look like:
generic
  type item_type is private;
  with package Hash is new hash_interface (hash_item_type => item_type);
  with package Tree is new tree_interface (tree_item_type => item_type);
package ...

But, of course, this declaration is incomplete, since if I already started 
supplying generic parameters, I have to specify them all, which is 
unwanted.

So, my question (finally!) is:
Is it possible to do what I want, or am I trying the wrong thing in Ada?

Thanks in advance,
Fingertip