sclass Borrowable<A> implements AutoCloseable {
  A object;
  bool borrowed;
  
  *(A *object) {}
  *() {}

  synchronized void set(A object) {
    this.object = object;
    borrowed = false;
  }
  
  synchronized bool canBorrow() { ret object != null && !borrowed; }
  
  synchronized bool isBorrowed() { ret borrowed; }
  
  // returns null if object is borrowed
  synchronized IAutoCloseableF0<A> tempBorrow() {
    bool canBorrow = canBorrow();
    A o = canBorrow ? object : null;
    if (canBorrow) borrowed = true;
    
    ret new IAutoCloseableF0<A> {
      public A get() { ret o; }
      public void close { unborrow(o); }
    };
  }
  
  synchronized void unborrow(A o) {
    if (o == object) borrowed = false;
  }
  
  public void close() {
    O o;
    synchronized(this) {
      o = object;
      object = null;
    }
    dispose o;
  }
}