Return the |NotesMasterPart| object for this presentation. If the
        presentation does not have a notes master, one is created from
        a default template. The same single instance is returned on each
        call.

def notes_master_part(self):
        """
        Return the |NotesMasterPart| object for this presentation. If the
        presentation does not have a notes master, one is created from
        a default template. The same single instance is returned on each
        call.
        """
        try:
            return self.part_related_by(RT.NOTES_MASTER)
        except KeyError:
            notes_master_part = NotesMasterPart.create_default(self.package)
            self.relate_to(notes_master_part, RT.NOTES_MASTER)
            return notes_master_part