121     head_.next_->Remove();  in ~ListHead()
127   head_.prev_->next_ = that;  in PushBack()
128   that->prev_ = head_.prev_;  in PushBack()
129   that->next_ = &head_;  in PushBack()
130   head_.prev_ = that;  in PushBack()
136   head_.next_->prev_ = that;  in PushFront()
137   that->prev_ = &head_;  in PushFront()
138   that->next_ = head_.next_;  in PushFront()
139   head_.next_ = that;  in PushFront()
144   return head_.IsEmpty();  in IsEmpty()
151   ListNode<T>* node = head_.next_;  in PopFront()
158   return Iterator(head_.next_);  in begin()
163   return Iterator(const_cast<ListNode<T>*>(&head_));  in end()