Return |PackURI| instance containing the partname for a slide to be
        appended to this slide collection, e.g. ``/ppt/slides/slide9.xml``
        for a slide collection containing 8 slides.

def _next_slide_partname(self):
        """
        Return |PackURI| instance containing the partname for a slide to be
        appended to this slide collection, e.g. ``/ppt/slides/slide9.xml``
        for a slide collection containing 8 slides.
        """
        sldIdLst = self._element.get_or_add_sldIdLst()
        partname_str = '/ppt/slides/slide%d.xml' % (len(sldIdLst)+1)
        return PackURI(partname_str)