/**
     * Notifies the group that the thread {@code t} has failed
     * an attempt to start.
     *
     * <p> The state of this thread group is rolled back as if the
     * attempt to start the thread has never occurred. The thread is again
     * considered an unstarted member of the thread group, and a subsequent
     * attempt to start the thread is permitted.
     *
     * @param  t
     *         the Thread whose start method was invoked
     *
     * @param  failed
     *         true if the thread could not be started successfully
     */

    void threadStartFailed(Thread t) {

  {- -------------------------------------------
  (1) java.lang.ThreadGroup.remove() で, 
      指定された Thread オブジェクトを threads フィールドから除去する 
      (同時に nthreads フィールドのデクリメントも行う).

      (ついでに, nUnstartedThreads フィールドの値をインクリメントも行っている)
      ---------------------------------------- -}

            synchronized(this) {
                remove(t);
                nUnstartedThreads++;
            }
        }