@Test public void testGroupingBy_groupedByDiff() {

  assertKeyValues(
          BiStream.of(1, 3, 2, 4, 11, 111)
              .collect(groupingBy((a, b) -> b - a, ImmutableSetMultimap::toImmutableSetMultimap)))
      .containsExactly(2, ImmutableSetMultimap.of(1, 3, 2, 4), 100, ImmutableSetMultimap.of(11, 111));
}