def merge(iterable_a, iterable_b):
    sentinel = object()

    a_iter = iter(iterable_a)
    b_iter = iter(iterable_b)

    a = next(a_iter, sentinel)
    b = next(b_iter, sentinel)

    while a is not sentinel and b is not sentinel:
        if a <= b:
            yield a
            a = next(a_iter, sentinel)
        else:
            yield b
            b = next(b_iter, sentinel)

    yield from a_iter
    yield from b_iter