class type ['a]
extension = object
.. end
method clone : < clone : 'b; node : 'a; set_node : 'a -> unit; .. > as 'b
"clone" should return an exact deep copy of the object.
method node : 'a
"node" returns the corresponding node of this extension. This method
intended to return exactly what previously has been set by "set_node".
method set_node : 'a -> unit
"set_node" is invoked once the extension is associated to a new
node object.