Null Methods

  A null method is a method which takes no action, and returns a value of
  NIL. A method can be declared null to suppress the behavior of that method
  in a subclass of the class in which it was originally defined, or to
  declare a method in a superclass which can be optionally overridden in a
  subclass.

  A null method is declared in a class specification as follows:

    MESSAGE <message name> IS NULL

  A method declared as NULL must not be defined in the class where it is
  declared. Any such definition will be ignored.