t.group = null;
  final synchronized void removeThread(Thread t)  {    if (groups == null)      return;    threads.remove(t);    // Daemon groups are automatically destroyed when all their threads die.    if (daemon_flag && groups.size() == 0 && threads.size() == 0)      {        // We inline destroy to avoid the access check.        groups = null;        if (parent != null)          parent.removeGroup(this);      }  }