def test_nested(): assert(flatten([[0]])) == [0]
def test_flat(): assert(flatten([0, 1, 2])) == [0, 1, 2]
def test_empty(): assert(flatten([])) == []
def test_tree_like(): assert(flatten([0, [1, [2, 3]]])) == [0, 1, 2, 3]
def test_doubly_nested_nested_and_flat(): assert(flatten([[0], 1, [[2]]])) == [0, 1, 2]
def test_nested_and_flat(): assert(flatten([[0], 1])) == [0, 1]