Hierarchy destruction in Ada

[gate02@wrandyke.demon.co.uk (Michael Mounteney)]

Ada 95 provides a mechanism with tagged and type'class to allow
dynamic dispatch. So if one has:

procedure something (O : basetype'class) is
begin
     enact (O);
end something;

then as we all know, with the appropriate redefinitions of enact ()
for the subtypes of basetype, we have dynamic dispatch.  But what
about deallocation ? I want to declare:

type handle is access all basetype;

procedure dispose (ptr : in out handle);

and can't see how to make this deallocate the correct object.
Obviously, one can dynamically dispatch on ptr.all but it seems
impossible to solve the complete problem without an unchecked
conversion. Anyone ?