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; }