method Ceiling7(n:nat) returns (k:nat)
requires n >= 0;
ensures k == n - (n % 7);
{
    return n - (n % 7);
}

method test() {
    var res := Ceiling7(0);
    assert(res == 0);

    res := Ceiling7(1);
    assert(res == 0);

    res := Ceiling7(6);
    assert(res == 0);

    res := Ceiling7(7);
    assert(res == 7);

    res := Ceiling7(8);
    assert(res == 7);

    res := Ceiling7(41);
    assert(res == 35);

    res := Ceiling7(42);
    assert(res == 42);

    res := Ceiling7(43);
    assert(res == 42);
}