Deferred Methods
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    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.

    A deferred method is declared (in a class declaration block) as
    follows:

        METHOD <method name> DEFERRED

    A method declared as DEFERRED must not be defined (using either METHOD
    FUNCTION or METHOD PROCEDURE) 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 all subclasses which may
    use the method.

    At present, if a deferred method is not implemented by a subclass, no
    action takes place when the message corresponding to the deferred
    method is sent.  This means that declaring a method deferred is
    equivalent to declaring it as a null method.  However, in the next
    version of Class(y), a runtime error will be generated if an attempt is
    made to invoke a deferred method that has not been implemented, which
    will not be the case for null methods.