(defclass shape ()
   ())
 
 (defclass positioned-obect ()
   ((%by :accessor by :initarg :by :initform 0)
    (%lx :accessor lx :initarg :lx :initform 0)
    (%shape :accessor shape :initarg :shape
            :initform (error "a positioned object needs a shape"))))
 
 (defgeneric lx (shape))
 (defgeneric by (shape))
 (defgeneric width (shape))
 (defgeneric height (shape))
 
 (defgeneric bounding-box (shape)
   (:documentation "Get the bounding box for a shape return a pair (#(LX BY) . #(W H))"))
 
 (defclass rect ()
   ((%width :accessor width :initarg width :initform 0)
    (%height :accessor height :initarg height :initform 0)))
 
 (defmethod bounding-box ((rect rect))
   (cons (vector (lx rect) (by rect))
         (vector (width rect) (height rect))))