Deferred Methods

  A deferred method is a method which is declared in a particular class, but
  only defined later in a subclass of that class. Declaring a method as
  deferred documents the fact that a class expects the specified method to
  be reimplemented by a subclass, and will cause a meaningful error to be
  generated if an attempt is made to invoke a deferred method that has not
  been redefined.

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

    MESSAGE <message name> IS DEFERRED

  A method declared as DEFERRED must not be defined in the same module
  as it is declared. Any such definition will be ignored. Rather, it should
  be both declared (as a normal method) and defined in subclasses of the
  class in which it is declared as deferred.