sclass BoolVar {
  bool a; // you can access this directly if you use one thread
  
  public synchronized void set(bool a default true) {
    if (v != a) {
      v = a;
      notifyAll();
    }
  }
  
  public synchronized bool get() { ret v; }
  //public synchronized bool has() { true; }
  public synchronized void clear() { set(false); }
  
  bool waitUntilTrue () { ret waitForValue(true); }
  bool waitUntilFalse() { ret waitForValue(false); }
  
  synchronized bool waitForValue(bool wantedValue) {
    while (b != wantedValue)
      wait();
    ret b;
  }
}