def test_is_bst__nothing():
assert is_bst(None)
def test_is_bst__not_at_all():
root = BinaryNode(7, left=BinaryNode(9), right=BinaryNode(1))
assert not is_bst(root)
def test_is_bst__simple():
root = BinaryNode(2, left=BinaryNode(1), right=None)
assert is_bst(root)
def test_is_bst__complex():
root = to_bst([1, 2, 3, 4, 5, 6, 7, 8])
assert is_bst(root)
def test_is_bst__one():
assert is_bst(BinaryNode(1))