(defclass sprite ()
  ((resource-keyword
    :initarg :resource-keyword
    :accessor sprite-resource-keyword
    :initform (error "Every sprite needs a resource keyword"))))


(defclass sprite-sheet-sprite (sprite)
  ((x :accessor x :initarg :x
      :initform (error "Every sprite-sheet-sprite needs an x-offset `x'."))
   (y :accessor y :initarg :y
      :initform (error "Every sprite-sheet-sprite needs a y-offset `y'."))
   (w :accessor w :initarg :w
      :initform (error "Every sprite-sheet-sprite needs a width `w'."))
   (h :accessor h :initarg :h
      :initform (error "Every sprite-sheet-sprite needs a height `h'."))
   (tile-size :accessor tile-size
              :initarg :tile-size
              :initform (error "Every sprite-sheet-sprite needs a `tile-size'."))
   (sprite-sheet :initarg :sprite-sheet
                 :initform (error "Every sprite-sheet-sprite needs a `sprite-sheet'")
                 :documentation "This is a keyword, which specifies which resource to refer to.")))

(defmethod initialize-instance :after ((sss sprite-sheet-sprite) &key)
  )


(defun make-sprite-sheet-sprite (x y w h tile-size sprite-sheet)
  (make-instance 'sprite-sheet-sprite
                 :x x :y y :w w :h h
                 :tile-size tile-size
                 :sprite-sheet sprite-sheet))