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

  public static <E> ImmutableList<E> of(E e1) {
    return new SingletonImmutableList<E>(checkNotNull(e1));
  }

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