for! enumerate(0..3), ((i, j),) =>
    assert i == j
for! zip(0..3, 0..3), ((i, j),) =>
    assert i == j
for! filter(x -> x > 1, 0..3), i =>
    assert i > 1
for! map(x -> "\{x}", [1, 2, 3]), s =>
    assert str(s) == s