sclass WeakVar<A> implements IVar<A> {
  WeakReference<A> ref;
  
  public synchronized void set(A a) {
    ref = weakRef(a);
  }
  
  public synchronized A get() {
    if (ref == null) null;
    A value = ref!;
    if (value == null) ref = null;
    ret value;
  }
}