code: def backlink(node):
      
      seen = set()
      to_see = [node]
      while to_see:
        node = to_see.pop()
        seen.add(node)
        for succ in node.next:
          succ.prev.add(node)
          if succ not in seen:
            to_see.append(succ)
docs: Given a CFG with outgoing links, create incoming links.