size_type size() const { return tracks_.size(); }

 bool empty() const { return tracks_.empty(); }

 const_iterator end() const { return tracks_.end(); }

 iterator erase(iterator i) { return tracks_.erase(i); }

 const_iterator begin() const { return tracks_.begin(); }

 void add(const track_ref &b)
 {
   tracks_.push_back(b);
 }

 iterator insert(iterator pos, const track_ref &b)
 {
   return tracks_.insert(pos, b);
 }

 iterator erase(const iterator &i) { return tracks_.erase(i); }