47     node* tail;  variable
52     Map() : head( NULL ), tail ( NULL ),tmp(head),size_of_list(0) {}  in Map()
53     bool empty() const { return ( !head || !tail ); }  in empty()
144     tail = new node(data, data2,tail, NULL);  in insert()
145     if( tail->prev )  in insert()
146         tail->prev->next = tail;  in insert()
150         head = tail;  in insert()
167         if((head == tail) && (head->data == d))  in erase()
171            head = tail = NULL;  in erase()
186         if((tmp == tail) && (tmp->data ==d))  in erase()
192             tail = prevnode;  in erase()
225     tail = head = NULL;  in eraseall()