What should be the initialiser for the label map?

[tommbendall]

What should be the initialiser for the label map?

We currently have:

NullTerm = Term(None)

    def label_map(self, term_filter, map_if_true=identity,
                  map_if_false=identity):

        new_labelled_form = LabelledForm(
            functools.reduce(operator.add,
                             filter(lambda t: t is not None,
                                    (map_if_true(t) if term_filter(t) else
                                     map_if_false(t) for t in self.terms)),
                             # TODO: Not clear what the initialiser should be!
                             # No initialiser means label_map can't work if everything is false
                             # None is a problem as cannot be added to Term
                             # NullTerm works but will need dropping ...
                             NullTerm))

        # Drop the NullTerm
        new_labelled_form.terms = list(filter(lambda t: t is not NullTerm,
                                              new_labelled_form.terms))

        return new_labelled_form

(the comments were included when the NullTerm was added and capture the problems pretty well!) The NullTerm is an OK solution to get everything working but doesn't seem very neat.