ClipProxy mutable_back() { return mutable_clip(size() - 1); }
const Clip *back() const { return clip(size() - 1); }
ClipProxy mutable_back() { return mutable_clip(size() - 1); }
const Clip *back() const { return clip(size() - 1); }
void set_currently_playing(int index, double progress); // -1 = none.
int get_currently_playing() const { return currently_playing_index; }
void set_currently_playing(int index, double progress); // -1 = none.
int get_currently_playing() const { return currently_playing_index; }