Iterator<? extends Iterator<? extends T>> inputs) {
    return new ConcatenatedIterator<>(inputs);
  }

  /** Concats a varargs array of iterators without making a defensive copy of the array. */
  static <T extends @Nullable Object> Iterator<T> concatNoDefensiveCopy(
      Iterator<? extends T>... inputs) {
    for (Iterator<? extends T> input : checkNotNull(inputs)) {