The pack URI of the .rels part corresponding to the current pack URI.
        Only produces sensible output if the pack URI is a partname or the
        package pseudo-partname '/'.

def rels_uri(self):
        """
        The pack URI of the .rels part corresponding to the current pack URI.
        Only produces sensible output if the pack URI is a partname or the
        package pseudo-partname '/'.
        """
        rels_filename = '%s.rels' % self.filename
        rels_uri_str = posixpath.join(self.baseURI, '_rels', rels_filename)
        return PackURI(rels_uri_str)