def test_type(self):
        rset = event.ResultSet()
        with pytest.raises(NotImplementedError):
            rset.extend([])
        with pytest.raises(NotImplementedError):
            rset.extend(False)
def dfs_print(root):
        if root is None:
            return
        dfs_print(root.left)
        print(str(root.key) + ' ', end='')
        dfs_print(root.right)