if (!first) {
        sb.append(", ");
      }
      first = false;
      sb.append(iterator.next());
    }
    return sb.append(']').toString();
  }

  /**
   * Returns the single element contained in {@code iterator}.
   *
   * @throws NoSuchElementException if the iterator is empty
   * @throws IllegalArgumentException if the iterator contains multiple elements. The state of the