def return_ys_unique(xs, ys):
    """ merge sorted lists xs and ys. Return only those items that are present in the second list,
    but not in the first. """
    result = []
    yi = 0

    while True:
        if yi >= len(ys):          # If xs list is finished,
            return result          # We're done.

        if ys[yi] not in xs:
            result.append(ys[yi])
            yi += 1

        else:
            yi += 1