*     that involve a null element being present.
   */
  protected int getNullLocation() {
    return getNumElements() / 2;
  }

  protected MinimalCollection<E> createDisjointCollection() {
    return MinimalCollection.of(e3(), e4());
  }

  protected MinimalCollection<E> emptyCollection() {
    return MinimalCollection.of();
  }

  protected final E e0() {