The required field is the factory field, of
class Implementation.
This tells how to construct a new EnhNode of this typ
from a Node. Note that caching has already happened, so
there is no point in implementing another cache here.
The code first checks that the interface is appropriate at this point.
This is not obligatory but should be considered.
(If the underlying graph has changed for the worse will
users prefer an early and unambiguous exception at this point).
See Also:com.hp.hpl.jena.enhanced.test.TestObject.aSubject