def get_irregular_vertex(bgedge):
    """
    This method is called only in irregular edges in current implementation, thus at least one edge will be irregular
    """
    if not bgedge.is_irregular_edge:
        raise Exception(
            'trying to retrieve an irregular vertex from regular edge')
    return (bgedge.vertex1 if bgedge.vertex1.is_irregular_vertex else
        bgedge.vertex2)
