private final String toString;

    protected Stimulus(String toString) {
      this.toString = toString;
    }

    /**
     * Send this stimulus to both iterators and return normally only if both produce the same
     * response.
     */
    abstract void executeAndCompare(ListIterator<E> reference, T target);

    @Override

    private final String toString;

    protected Stimulus(String toString) {
      this.toString = toString;
    }

    /**
     * Send this stimulus to both iterators and return normally only if both produce the same
     * response.
     */
    abstract void executeAndCompare(ListIterator<E> reference, T target);

    @Override