<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _add_notes_slide_part(self):
 """ 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. """

        notes_slide_part = NotesSlidePart.new(self.package, self)
        self.relate_to(notes_slide_part, RT.NOTES_SLIDE)
        return notes_slide_part