interface IG22MasksHolder<A> {
  replace Mask with IG22Mask<A>.
  
  WidthAndHeight maskSize();
  L<Image2B> maskImages();
  //LPair<Image2B, A> maskImagesWithLabels();
  L<? extends Mask> masks();
  FloatBWImage ghost();
  double certainty();
  default int nMasks() { ret l(masks()); }
  
  default S renderCountAndCertainty() {
    ret main nMasks(nMasks()) + " with c=" + formatDouble3(certainty());
  }
  
  default bool canSplit() { ret nMasks() > 1; }
  
  default L<IG22MasksHolder<A>> subHolders() { null; }
  default void transformSubHolders(IF1<IG22MasksHolder<A>> f) {}
  
  default PStackComputable<Void> findSimilarMask(
    BWImage maskImage, ProbabilisticList<Scored<Mask>> outList) {
    ret new FindSimilarMaskSimple<A>(this, maskImage, outList);
  }

  srecord noeq FindSimilarMaskSimple<A>(IG22MasksHolder masksHolder,
    Image2B maskImage,
    ProbabilisticList<Scored<Mask>> outList) extends PStackComputableWithStep<Void> {
    Mask currentMask;
    
    void step(IPStack stack) {
      if (step == 0) {
        ++step;
        stack.options(this, map(masksHolder.masks(), mask ->
          self -> self.currentMask = mask));
      } else {
        long diff = bwRawImageDiff_sameSize(
          bwImage(maskImage),
          bwImage(mask.image()));
        double p = 1-doubleRatio(diff, area(mask.image())*255);
        outList.at(p, currentMask);
        stack.ret();
      }
    }
  }
}