Delegation Constraint Added

In the process of coding up the API, I've discovered a missing constraint in the Polymorphism Subsystem.  Due to rule #4 in Mellor-Balcer regarding polymorphism, it is critical that delegation of a polymorphic event proceed down exactly one Generalization.  This is important when we have a compound generalization where multiple hierarchies sprout from the same Class.  I will be posting an updated model by the close of the weekend with this constraint added.