40     T* tail = nullptr;  member
49         head = tail = nullptr;  in free()
54         if (tail) {  in back()
55             tail->next = element;  in back()
56             element->prev = tail;  in back()
58             tail = element;  in back()
60             head = tail = element;  in back()
74             head = tail = element;  in front()
82         if (!tail) return nullptr;  in back()
83         auto t = tail;  in back()
84         tail = t->prev;  in back()
85         if (!tail) head = nullptr;  in back()
94         if (!head) tail = nullptr;  in front()
103         if (element == tail) tail = element->prev;  in remove()