@abstractmethod<EOL><INDENT>def prune(self, model):<DEDENT>
raise NotImplementedError()<EOL>
Reduce the classifier set's population size, if necessary, by
        removing lower-quality *rules. Return a list containing any rules
        whose numerosities dropped to zero as a result of this call. (The
        list may be empty, if no rule's numerosity dropped to 0.) The
        model argument is a ClassifierSet instance which utilizes this
        algorithm.

        Usage:
            deleted_rules = model.algorithm.prune(model)

        Arguments:
            model: A ClassifierSet instance whose population may need to
                be reduced in size.
        Return:
            A possibly empty list of ClassifierRule instances which were
            removed entirely from the classifier set because their
            numerosities dropped to 0.