The |NotesSlide| instance associated with this slide. If the slide
        does not have a notes slide, a new one is created. The same single
        instance is returned on each call.

def notes_slide(self):
        """
        The |NotesSlide| instance associated with this slide. If the slide
        does not have a notes slide, a new one is created. The same single
        instance is returned on each call.
        """
        try:
            notes_slide_part = self.part_related_by(RT.NOTES_SLIDE)
        except KeyError:
            notes_slide_part = self._add_notes_slide_part()
        return notes_slide_part.notes_slide