sclass Trigger implements Runnable {
  bool triggered;
  L<Runnable> onTrigger;
  
  *() {}
  *(Runnable onTrigger) { this.onTrigger = ll(onTrigger); }
  
  // for "user"
  synchronized void trigger() {
    synchronized(this) {
      if (triggered) ret;
      triggered = true;
    }
    pcallFAll(onTrigger);
  }
  
  public void run() { trigger(); }
  
  // for "implementer" - check if triggered and clear flag
  synchronized bool check() {
    if (!triggered) false;
    ret true with triggered = false;
  }
}