docs: public static <T, C extends Collection<T>> Map<T, C> makeElementsToContainersMap(
        final Iterable<C> collections) {
      final ImmutableMap.Builder<T, C> ret = ImmutableMap.builder();
  
      for (final C collection : collections) {
        for (final T item : collection) {
          ret.put(item, collection);
        }
      }
  
      return ret.build();
    }