docs: public static <K> ImmutableSet<K> allMultimapKeys(final Iterable<? extends Multimap<K, ?>> multimaps) {
      final ImmutableSet.Builder<K> builder = ImmutableSet.builder();
  
      for (final Multimap<K, ?> multimap : multimaps) {
        builder.addAll(multimap.keySet());
      }
  
      return builder.build();
    }