def is_bull_free(G):
    # define a bull graph, also known as the graph obtained from the complete graph K_3 by addiing two pendants
    bull = gp.Graph([(0, 1), (0, 2), (1, 2), (1, 3), (2, 4)])

    # enumerate over all possible combinations of 5 vertices contained in G
    for S in set(itertools.combinations(G.nodes(), 5)):
        H = G.subgraph(list(S))
        if gp.is_isomorphic(H, bull):
            return False
    # if the above loop completes, the graph is bull-free
    return True