code: def _add_notes_slide_part(self):
          
          notes_slide_part = NotesSlidePart.new(self.package, self)
          self.relate_to(notes_slide_part, RT.NOTES_SLIDE)
          return notes_slide_part
docs: Return a newly created |NotesSlidePart| object related to this slide
          part. Caller is responsible for ensuring this slide doesn't already
          have a notes slide part.