sclass Ext<A> extends DynamicObject implements IVar<A> {
  *() {}
  *(A *v) {}
  //O equalityPredicate; // func(Ext, Ext) -> bool
  
  bool fullToString; // include values in toString

  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 synchronized void clear() { v = null; }
  
  public bool equals(O o) {
    ret o instanceof Ext && eq(o/Ext.v, v);
  }
  
  public int hashCode() { ret _hashCode(v); }
  
  toString {
    ret !fullToString ? str(get()) : extToString(this);
  }
}