def sum_exists_set(a, b, target):
    """
    >>> sum_exists_set([1, 2], [3, 4], target=6)
    True
    >>> sum_exists_set([1, 2], [3, 4], target=7)
    False
    """
    b_remainders = set()
    for el in a:
        b_remainders.add(target - el)

    for el in b:
        if el in b_remainders:
            return True
    return False