code: def link_tail(self, node):
          '''Add a node to the tail.'''
          assert not node.head
          old_tail = self.tail
  
          if old_tail:
              assert old_tail.head == self
              old_tail.head = node
              node.tail = old_tail
  
          node.head = self
          self.tail = nodf link_tail(self, node):
          '''Add a node to the tail.'''
          assert not node.head
          old_tail = self.tail
  
          if old_tail:
              assert old_tail.head == self
              old_tail.head = node
              node.tail = old_tail
  
          node.head = self
          self.tail = node
docs: Add a node to the tail.