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); }
  }
  
  void add(A a, A prev default null) {
    if (!links.containsKey(a)) {
      links.put(a, prev);
      queue.add(a);
    }
  }
  
  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;
  }
  
  bool nodeReached(A a) {
    ret links.containsKey(a);
  }
  
  Cl<A> getBackLinks(A a) {
    ret links.get(a);
  }
  
  L<A> examplePath(A start, A dest) {
    new L<A> path;
    A node = dest;
    path.add(node);
    do {
      if (node == null) null; // no path found
      node = first(getBackLinks(node));
      path.add(node);
    } while (node != start);
    ret reversed(path);
  }
}