template <class T>
class dll_t {
private:
 dll_node_t<T>* head_;
 dll_node_t<T>* tail_;
 int sz_;
public:
 dll_t(void);
 ~dll_t(void);
void insert_tail(dll_node_t<T>*);
 void insert_head(dll_node_t<T>*);
 dll_node_t<T>* extract_tail(void);
 dll_node_t<T>* extract_head(void);
 dll_node_t<T>* get_tail(void) const;
 dll_node_t<T>* get_head(void) const;
 bool empty(void);
 void remove(dll_node_t<T>*);
 ostream& write(ostream& os) const;
 void select(dll_t<dll_node_t<T>*>& L, const T& x);
private:
bool igual(const T& a, const T&b);
};

template <class T>
class dll_node_t {
private:
 dll_node_t<T>* next_;
 dll_node_t<T>* prev_;
T data_;
public:
 dll_node_t();
dll_node_t(const T& data);
 virtual ~dll_node_t(void);
void set_next(dll_node_t<T>*);
void set_prev(dll_node_t<T>*);
dll_node_t<T>* get_next(void) const;
dll_node_t<T>* get_prev(void) const;
void set_data(const T& data);
T get_data(void) const;
ostream& write(ostream& os) const;
};

template<class T>

   ostream& dll_t<T>:: write_reverse(dll_node_t<T>* n, ostream& os) const {
     assert(!empty());
     while(n!=NULL){
       n=n->get_data();
       n.write(os);
   }