def verts_in_common(self, segments):
        """
        returns array of all vertex indices common to each segment in segments"""
        verts_by_segm = self.verts_by_segm
        return sorted(reduce(lambda s0, s1: s0.intersection(s1),
                             [set(verts_by_segm[segm]) for segm in segments]))
returns array of all vertex indices common to each segment in segments