def _enclose_cit_rel(net):
    """
    add sink and source to a citation network
    args:
        net: an unweighted citation network
    return:
        enclosing: list of edges to add sink and source
    """

    try:
        source_edges = [('source', node, {'weight': 1}) for node in net.nodes if net.in_degree(node) == 0]
        sink_edges = [(node, 'sink', {'weight': 1}) for node in net.nodes if net.out_degree(node) == 0]
        return(source_edges + sink_edges)
    except:
        return(False)