sclass HashedBWImage {
  BWImage image;
  long hash;
  
  long hash() { if (hash == 0) hash = hashBWImage(image); }
  
  equals HashedBWImage o {
    ret hash() == o.hash() && bwImagesIdentical(image, o.image);
  }
  
  public int hashCode() { ret (int) hash; }
}