code: def node_is_on_list(self, node):
          
          next = self.node_next(node)
          if next == node or next is None:
              assert(self.node_prev(node) is next)
              return False
  
          return True
docs: Returns True if this node is on *some* list.
  
          A node is not on any list if it is linked to itself, or if it
          does not have the next and/prev attributes at all.