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);

  }

}