code: def add_slide(self, slide_layout):
          
          partname = self._next_slide_partname
          slide_layout_part = slide_layout.part
          slide_part = SlidePart.new(partname, self.package, slide_layout_part)
          rId = self.relate_to(slide_part, RT.SLIDE)
          return rId, slide_part.slide
docs: Return an (rId, slide) pair of a newly created blank slide that
          inherits appearance from *slide_layout*.