code: func (d Dir) String() string {
  	switch d {
  	case DirUp:
  		return "Up"
  	case DirRight:
  		return "Right"
  	case DirDown:
  		return "Down"
  	case DirLeft:
  		return "Left"
  	}
  	panic("not reach")
  }
docs: // String returns a string representing the direction.