code: def makeDoubleLinked(dom, parent=None):
      
      dom.parent = parent
  
      for child in dom.childs:
          child.parent = dom
          makeDoubleLinked(child, dom)
docs: Standard output from `dhtmlparser` is single-linked tree. This will make it
      double-linked.
  
      Args:
          dom (obj): :class:`.HTMLElement` instance.
          parent (obj, default None): Don't use this, it is used in recursive
                 call.