def test_subset(self):
        t = self.simple_t
        self.assertEqual(t.subset(), frozenset('abcd'))
        c = t.children[0]
        self.assertEqual(c.subset(), frozenset('ab'))
        leaf = c.children[1]
        self.assertEqual(leaf.subset(), frozenset(''))
subset should return set of leaves that descends from node
dataset/ETHPy150Open biocore/scikit-bio/skbio/tree/tests/test_tree.py/TreeTests.test_subset