39     T* head = nullptr;  member
44         while (head) {  in free()
45             auto t = head;  in free()
46             head = t->next;  in free()
49         head = tail = nullptr;  in free()
60             head = tail = element;  in back()
68         if (head) {  in front()
69             head->prev = element;  in front()
71             element->next = head;  in front()
72             head = element;  in front()
74             head = tail = element;  in front()
85         if (!tail) head = nullptr;  in back()
91         if (!head) return nullptr;  in front()
92         auto t = head;  in front()
93         head = t->next;  in front()
94         if (!head) tail = nullptr;  in front()
102         if (element == head) head = element->next;  in remove()
108         return head ? false : true;  in empty()