@SuppressWarnings("unchecked")
  public static <E> ImmutableList<E> of() {
    return (ImmutableList<E>) RegularImmutableList.EMPTY;
  }

  public static <E> ImmutableList<E> of(E element) {
    return new SingletonImmutableList<E>(checkNotNull(element));
  }

  public static <E> ImmutableList<E> of(E e1, E e2) {
    return new RegularImmutableList<E>(ImmutableList.<E>nullCheckedList(e1, e2));
  }

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

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