docs: public static <K, V> ImmutableSet<K> allKeys(final Iterable<? extends Map<K, V>> maps) {
      final ImmutableSet.Builder<K> builder = ImmutableSet.builder();
  
      for (final Map<K, V> map : maps) {
        builder.addAll(map.keySet());
      }
  
      return builder.build();
    }