*
   * @since 10.0 (present in 1.0 as {@code run})
   */
  public void execute() {
    // Lock while we update our state so the add method above will finish adding any listeners
    // before we start to run them.
    RunnableExecutorPair list;
    synchronized (this) {
      if (executed) {
        return;
      }
      executed = true;
      list = runnables;

        return Iterators.forArray(1, 2, 3);
      }
    }.test();
  }

  /*
   * TODO(cpovirk): Test forArray with ListIteratorTester (not just IteratorTester), including with
   * a start position other than 0.
   */

  public void testForEnumerationEmpty() {
    Enumeration<Integer> enumer = enumerate();
    Iterator<Integer> iter = Iterators.forEnumeration(enumer);

    /*
     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
     * is the only one in use, but if we encounter nested concatenations, we start a deque of
     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
     * operation O(1).
     */