anchor.diff:  ;;------------------------------------------------------------
   ;; newtonslidercallback
   
  -(defcallback %slider-cb :void
  +(defcallback %slider-cb :uint
       ((joint-ptr :pointer)
        (data (:pointer NewtonHingeSliderUpdateDesc)))
     (with-foreign-slots ((m-accel m-minFriction m-maxFriction m-timestep)
  
positive.code: ;;------------------------------------------------------------
  ;; newtonhingecallback
  
  (defcallback %hinge-cb :void
      ((joint-ptr :pointer)
       (data (:pointer NewtonHingeSliderUpdateDesc)))
    (with-foreign-slots ((m-accel m-minFriction m-maxFriction m-timestep)
  
negative.code: 
  ;; newtontreecollisioncallback
  
  (defcallback %geom-tree-face-cb :void ((tree-ptr :pointer)
                                         (body-ptr :pointer)
                                         (face-id :int)
                                         (vertex-count :int)
                                         (vertex-arr :pointer)
                                          (stride-in-bytes :int))
    (let ((tree (%geom-ptr->geom tree-ptr))
          (body (%body-ptr->body body-ptr))
  