anchor.diff:  
   (defun (setf body-force-torque-callback) (callback body)
     (newtonbodysetforceandtorquecallback
  -   (%body-ptr body) '%body-apply-force-and-torque)
  +   (%body-ptr body) (get-callback '%body-apply-force-and-torque))
     (setf (%body-force-torque-callback body) callback))
   
   ;;------------------------------------------------------------
  
positive.code:   (with-foreign-array (v3 force-vec3 '(:array :float 3))
      (newtonbodyaddforce (%body-ptr body) v3)))
  
  (defun body-add-force (body delta-vec3 &optional (position-vec3 (v! 0 0 0)))
    (with-foreign-array (d3 delta-vec3 '(:array :float 3))
      (with-foreign-array (p3 position-vec3 '(:array :float 3))
        (newtonbodyaddimpulse (%body-ptr body) d3 p3))))
  
negative.code:   ;; {TODO} what does state mean
    (newtonbodysetsimulationstate (%body-ptr body) state))
  
  (defun body-get-skeleton (body)
    ;; {TODO} how are we meant to handle skeletons?
    (newtonbodygetskeleton (%body-ptr body)))
  
  