52     void back(T* element)  in back()
55             tail->next = element;  in back()
56             element->prev = tail;  in back()
57             element->next = nullptr;  in back()
58             tail = element;  in back()
60             head = tail = element;  in back()
61             element->prev = nullptr;  in back()
62             element->next = nullptr;  in back()
66     void front(T* element)  in front()
69             head->prev = element;  in front()
70             element->prev = nullptr;  in front()
71             element->next = head;  in front()
72             head = element;  in front()
74             head = tail = element;  in front()
75             element->prev = nullptr;  in front()
76             element->next = nullptr;  in front()
98     void remove(T* element)  in remove()
100         if (element->prev) element->prev->next = element->next;  in remove()
101         if (element->next) element->next->prev = element->prev;  in remove()
102         if (element == head) head = element->next;  in remove()
103         if (element == tail) tail = element->prev;  in remove()