persistable sclass G22PixelSplitMasks<A> is IG22OptimizedMasksHolder<A> {
  replace Mask with IG22Mask<A>.
  
  settable Pt splitPixel;
  settable IG22MasksHolder<A> darkHolder;
  settable IG22MasksHolder<A> brightHolder;
  
  // for search - how much the "correct" branch is preferred
  settable double bias = 0.9;
  
  public L<Mask> masks() {
    ret new ConcatOnDemandList(
      darkHolder.masks(),
      brightHolder.masks());
  }
  
  public L<IG22MasksHolder<A>> subHolders() {
    ret ll(darkHolder, brightHolder);
  }
  
  public void transformSubHolders(IF1<IG22MasksHolder<A>> f) {
    darkHolder = f.get(darkHolder);
    brightHolder = f.get(brightHolder);
  }
  
  @Override
  public PStackComputable<Void> findSimilarMasks(G22FindSimilarMasksTask<A> task) {
    ret new FindSimilarMasks(task);
  }

  record noeq FindSimilarMasks(G22FindSimilarMasksTask task) extends PStackComputableWithStep<Void> is IMakeEmptyClone {
    bool brightBranch;
    
    public selfType makeEmptyClone() { ret new selfType; }
    
    void step(IPStack stack) {
      if (task.ended()) ret with stack.ret();
      
      if (step == 0) {
        ++step;
        bool bright = isMaskBright(task.queryImage);
        if (task.verbose()) printVars("step 0", +splitPixel, +bright);
        stack.probabilisticOptions(this, ll(
          withProbability(1.0,    self -> self.brightBranch = bright),
          withProbability(1-bias, self -> self.brightBranch = !bright)));
      } else {
        if (task.verbose()) printVars("step 1", +splitPixel, +brightBranch);
        IG22MasksHolder<A> branch = brightBranch ? brightHolder : darkHolder;
        stack.tailCall(branch.findSimilarMasks(task));
      }
    }
  }
  
  // Does the mask belong in darkHolder or brightHolder?
  bool isMaskBright(Image2B maskImage) {
    ret maskImage.getBoolPixel(splitPixel);
  }
  
  public <B> IG22MasksHolder<B> cloneTreeWithLabelTransform(IF1<A, B> f) {
    ret new G22PixelSplitMasks().splitPixel(splitPixel)
      .darkHolder(darkHolder.cloneTreeWithLabelTransform(f))
      .brightHolder(brightHolder.cloneTreeWithLabelTransform(f));
  }
  
  !include early #1035854 // Masks holder Include
}

Began life as a copy of #1035850