anchor.diff:  (defmethod update-aabb ((collision-rectangle collision-rectangle) mat4 translation-vec3)
     (let ((new-center-point (vec3 0.0))
   	(new-radius (vec3 0.0)))
  -    (with-slots (center-point radius) collision-rectangle
  +    (with-slots (center-point radius radians) collision-rectangle
         (macrolet ((c (subscript) `(aref new-center-point ,subscript))
   		 (r (subscript) `(aref new-radius ,subscript))
   		 (t (subscript) `(aref translation-vec3 ,subscript)))
  
positive.code: 
  (defclass collision-rectangle ()
    ((center-point :initarg :center-point :type vec3)
     (radius :initarg :radius :type vec3)))
  
  
  (defgeneric set-radius (collision-rectangle radius)
  
negative.code:       (values new-center-point
  	      new-radius))))
  
  ;; NEXT-TODO:
  (defmethod rotate-bounding-volume ((collision-rectangle collision-rectangle) rotation-vec3)
    (with-slots (center-point radius radians) collision-rectangle
      (incf-radians radians (aref rotation-vec3 2)) ;; get the z-value (we assume we only ever rotate
                                                    ;; about the z-axis ..
      (multiple-value-bind (new-center-point new-radius)
  	(update-aabb collision-rectangle (sb-cga:rotate rotation-vec3) (vec3 0.0 0.0 0.0))
        ;; keeps growing, just as the caveat mentioned
        (setf radius new-radius)
        )))
  
  
  