Generic formal access types

Does anybody know why Ada does not have something like:

generic
    type Access_Type is access (<>);  --  ?

I would like to specify in a generic package that it should be 
instantiated by an access type, no matter what type it is pointing to. 	

Ok, it is not very useful to have an access type if you do not know its 
content, but the package implements a list and I am just storing them. I 
"need" that because I want to return the value "null" from a function in 
the generic package that returns "Access_Type".

There is a possible workaround:

generic
    type Element is (<>);
    type Access_Type is access Element;

But I do not like it because it forces me to pass also the "Element" in 
the instantiation.

Any support for a possible proposal?

Rodrigo