Null Methods
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    A null method is a method which takes no action, and returns a value of
    NIL.  A method can be declared as NULL to suppress the behavior of
    that method in a subclass.

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

        METHOD <method name> NULL

    A method declared as NULL must not be defined (using either METHOD
    FUNCTION or METHOD PROCEDURE) in the class where it is declared.  Any
    such definition will be ignored.