code: public static <K, V> ImmutableSet<V> getAllAsSet(final Multimap<K, V> multimap,
        final Iterable<K> keys) {
      final ImmutableSet.Builder<V> ret = ImmutableSet.builder();
      for (final K key : keys) {
        ret.addAll(multimap.get(key));
      }
      return ret.build();
    }
docs: Returns an {@link ImmutableSet} of all items in the multimap for the given set of keys, without
  preserving mulitiplicity.