sclass HashedBWImage {
  BWImage image;
  long hash;
  
  long hash() {
    if (hash == 0) {
      hash = hashBWImage(image);
      if (hash == 0) reportHashCollision(this);
    }
    ret hash;
  }
  
  equals HashedBWImage o {
    if (hash() != o.hash()) false;
    if (!bwImagesIdentical(image, o.image)) {
      reportHashCollision(this, o);
      false;
    }
    true;
  }
  
  public int hashCode() { ret (int) hash; }
  
  void hashCollision() {}
}