SetMultimap<K, V> filterKeys(
          SetMultimap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    if (unfiltered instanceof FilteredKeySetMultimap) {
      FilteredKeySetMultimap<K, V> prev = (FilteredKeySetMultimap<K, V>) unfiltered;
      return new FilteredKeySetMultimap<>(
          prev.unfiltered(), Predicates.<K>and(prev.keyPredicate, keyPredicate));
    } else if (unfiltered instanceof FilteredSetMultimap) {