srecord noeq G22SkeletonWalker<Img>(IImageRegion<Img> region) is Steppable {
  // nodes by starting point (filled during exploration)
  new Map<Pt, Node> nodesByPt;
  
  // temporary data
  
  ItIt<Pt> ptIterator; // region pixel iterator
  new LinkedList<Pair<Node, Pt>> queue; // incomplete nodes queue (with point to check)
  
  // each node is a path, at the end of which there is an optional
  // junction with multiple branches
  class Node {
    new PtBuffer path;
    L<Node> branches; // junction part (optional)
     
    Pt startingPt() { ret first(path); }
  }
  
    public bool step() {
    ping();
    
    if (nempty(queue)) {
      Pt p = popFirst(queue);
    }
    
    if (ptIterator == null)
      ptIterator = region.pixelIterator();
      
    fOr (A b : getChildren.get(a))
      add(b, a);
    true;
  }
}