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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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