docs: public static <K1, K2, V1 extends K2, V2> ImmutableSetMultimap<K1, V2> composeToSetMultimap(
        final Multimap<K1, V1> first, final Multimap<K2, V2> second) {
      final ImmutableSetMultimap.Builder<K1, V2> result = ImmutableSetMultimap.builder();
      for (K1 k1 : first.keySet()) {
        checkNotNull(k1);
        for (V1 v1 : first.get(k1)) {
          checkNotNull(v1);
          result.putAll(k1, second.get(v1));
        }
      }
      return result.build();
    }