srecord noeq G22SkeletonWalker<Img extends WidthAndHeight>(IImageRegion<Img> region) is Steppable {
  event foundLink(Pt a, Pt b);
  
  // point to "registered" point close by
  new Map<Pt> ptMap;
  
  // TEMPORARY DATA
  
  bool initialized;
  
  // each pair is (registered point, point-to-explore)
  new LinkedList<Pair<Pt>> queue;
  
  public bool step() {
    // find first pixel
    if (!initialized) {
      set initialized;
      Pt p = region.firstPixel();
      if (p == null) false;
      queue.add(pair(null, p));
    }
      
    if (empty(queue)) false;
    
    Pair<Pt> pair = popFirst(queue);
    Pt prev = pair.a, p = pair.b;
    p = lookupOrKeep(ptMap, p);
    
    if (pair.a != null) {
      if (distantEnough(prev, p))
        foundLink(prev, p);
      else
        ptMap.put(p, prev);
    }

    if (!ptMap.containsKey(p)) {
      ptMap.put(p, p);
      for (int dir = 1; dir <= 8; dir++) {
        Pt p2 = ptPlus(p, onePathDirection(dir));
        if (region.contains(p2))
          queue.add(pair(or(prev, p), p2));
      }
    }
    
    true;
  }
  
  bool distantEnough(Pt a, Pt b) {
    ret a != null && b != null
      && (absDiff(a.x, b.x) >= 2 || absDiff(a.y, b.y) >= 2);
  }
}