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(); }
iterator end() { return tracks_.end(); }
iterator begin() { return tracks_.begin(); }
void add(const track_ref &b) { tracks_.push_back(b); }
iterator erase(iterator a, iterator b) { return tracks_.erase(a, b); }
iterator insert(iterator pos, const track_ref &b) { return tracks_.insert(pos, b); }
iterator erase(const iterator &i) { return tracks_.erase(i); }