def test_subsets(self):
        t = self.simple_t
        self.assertEqual(t.subsets(), frozenset(
            [frozenset('ab'), frozenset('cd')]))
subsets should return all subsets descending from a set
dataset/ETHPy150Open biocore/scikit-bio/skbio/tree/tests/test_tree.py/TreeTests.test_subsets