@Test public void imidazole() throws Exception {

    Graph g = Graph.fromSmiles("[nH]1ccnc1");
    Matching m = Matching.empty(g);
    MaximumMatching.maximise(g,
                             m,
                             0,
                             IntSet.allOf(1, 2, 3, 4)); // not the 'nH'
    assertThat(m.matches(), hasItems(Tuple.of(1, 2),
                                     Tuple.of(3, 4)));
}