def get_cited_cases(G, citing_vertex):
    """
    Returns the ciations of a cases whose cited year
    is strictly less than citing year

    Parameters
    ----------
    G: igraph object

    citing_vertex: igraph vertex

    Output
    ------
    list of CL ids of cited cases
    """

    # get neighbors first as ig index
    all_citations = G.neighbors(citing_vertex.index, mode='OUT')

    # return CL indices of cases
    # only return cited cases whose year is stictly less than citing year
    return [G.vs[ig_id]['name'] for ig_id in all_citations
            if G.vs[ig_id]['year'] < citing_vertex['year']]