depth property
      
      int
      get
      depth
      
    
    
    
Implementation
int get depth {
  int d = 0;
  SpanNode? p = parent;
  while (p != null) {
    p = p.parent;
    if (p != null && p is UlOrOLNode && _listTag.contains(p.tag)) d += 1;
  }
  return d;
}