Partial Polymorphism

  In most uses of delegation other than simple message forwarding, it is
  likely that something short of full polymorphism between the delegator and
  delegatee will be required. In these cases, message sends to SENDER() in
  the delegatee class will be mixed with message sends to self, as
  appropriate.

  Warning

  When writing such code, care should be taken to avoid the incorrect
  invocation of methods which assume that they are being invoked via
  delegation. The SENDER() function will only be valid if such methods are
  in fact invoked via delegation. Invoking such methods in another way can
  lead to endless recursion and other aberrant behavior. In future verions,
  the SENDER() function may be changed to return NIL if it is invoked in an
  incorrect context, so be aware of this.