problems with generic package

[user@domain.invalid]

good morning

i have declared a generic package generic_set with the following 
specification:


generic
    type Element is private;

package Generic_Set is
...
end package;



then in another package i introduced a tagged type with two derived 
types like this:


type node is tagged null record;
type place is new node with record ... end record;
type transition is new node with record ... end record;


how is it possible to declare a new package set over the node type?
this declaration doesn't work :

package nodes_set is new generic_set(element => node'class);

error:

instantiation error
unconstrained subtype in component declaration
actual for "Element" must be a definite subtype


thanks for any help