Return a reference to a newly created <p:sldId> child element having
        its r:id attribute set to *rId*.

def add_sldId(self, rId):
        """
        Return a reference to a newly created <p:sldId> child element having
        its r:id attribute set to *rId*.
        """
        return self._add_sldId(id=self._next_id, rId=rId)