MetaClass extends the MOP interface with methods which have a senderClass as their first parameter. Unfortunately, the param does not refer to the sender of the original method call but is in most cases identical to the receiver's class.
Knowing the sender of a method call would allow interesting reflective program structures like subjective programming with a three-dimensional message dispatch. A method can not only be overwritten by the receiver with polymorphism but also by the sender. Refer to http://citeseer.ist.psu.edu/smith96simple.html for more details.