Return an (rId, slide) pair of a newly created blank slide that
        inherits appearance from *slide_layout*.

def add_slide(self, slide_layout):
        """
        Return an (rId, slide) pair of a newly created blank slide that
        inherits appearance from *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