* preferable mainly for consistency and maintainability of your code.
   *
   * @throws NullPointerException if the element is null
   */
  public static <E> ImmutableList<E> of(E e1) {
    return new SingletonImmutableList<>(e1);
  }

  /**
   * Returns an immutable list containing the given elements, in order.
   *
   * @throws NullPointerException if any element is null
   */