@Override
    public Range<T> span() {
        if (rangeBitSetMap.isEmpty()) {
            return null;
        }
        Entry<Long, BitSet> firstSet = rangeBitSetMap.firstEntry();
        Entry<Long, BitSet> lastSet = rangeBitSetMap.lastEntry();
        int first = firstSet.getValue().nextSetBit(0);
        int last = lastSet.getValue().previousSetBit(lastSet.getValue().size());
        return Range.openClosed(consumer.apply(firstSet.getKey(), first - 1), consumer.apply(lastSet.getKey(), last));
    }
@Test
    public void testNPE() {
        OpenLongPairRangeSet<LongPair> set = new OpenLongPairRangeSet<>(consumer);
        assertNull(set.span());
    }