A v; // you can access this directly if you use one thread

public synchronized void set(A a) {

  if (v != a) {

    v = a;

    notifyAll();

  }

}

public synchronized A get() { ret v; }

public synchronized bool has() { ret v != null; }

public void clear() { set(null); }

public synchronized A getAndSet(A a) {

  var value = v;

  set(a);

  ret value;

}

public IF0<A> getter() { ret l0 get; }

public IVF1<A> setter() { ret l1 set; }