package draw9 type Screen struct {	Display *Display	ID      uint32	Image   *Image	Fill    *Image} func (s *Screen) Free() error {	s.Display.mu.Lock()	defer s.Display.mu.Unlock()	return s.free()} func (s *Screen) free() error {	if s == nil {		return nil	}	d := s.Display	a := d.bufimage(1 + 4)	a[0] = 'F'	bplong(a[1:], s.ID)	// flush(true) because screen is likely holding the last reference to window,	// and we want it to disappear visually.	return d.flush(true)}