* elements of each iterator in {@code inputs}. The input iterators are not polled until
   * necessary.
   *
   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
   * supports it. The methods of the returned iterator may throw {@code NullPointerException} if any
   * of the input iterators is null.
   */
  public static <T extends @Nullable Object> Iterator<T> concat(