@Override
    public boolean containsAll(Collection<?> c) {
        for (Object o : c) {
            if (!contains(o)) return false;
        }
        return true;
    }
@Test
    void testContainsAll() {
        RangeSet rangeSet = new RangeSet(5, 10);
        assertTrue(rangeSet.containsAll(mkSet(5, 6, 7, 8, 9)));
        assertFalse(rangeSet.containsAll(mkSet(5, 6, 10)));
    }