anchor.diff:  
   (defmethod combine-physics-objects ((this geometry) (that geometry) params)
   
  -    (with-accessors ((mode surface-parameters-mode)
  -		     (bounce surface-parameters-bounce)
  -		     (bounce-vel surface-parameters-bounce-vel)) params
  +    (cffi:with-foreign-slots ((mode mu mu2 rho rho2 rhoN bounce bounce-vel soft-erp soft-cfm motion1 motion2 motionN slip1 slip2)
  +			      params
  +			      (:struct surface-parameters-struct))
         
  +
         (setf mode
   	    (union (surface-Mode this) (surface-Mode this)))
  +
         
  -      (setf (surface-parameters-mu params)
  -	    (* (surface-mu this) (surface-mu that)))
  +      
  +      (setf mu       
  +	    (max (surface-mu this) (surface-mu that)))
         
   	(when (member :mu2 mode)
  -	  (setf (surface-parameters-mu2 params)
  +	  (setf mu2
   		(* (surface-mu2 this) (surface-mu2 that))))
   	
   	(when (member :bounce mode)
  
positive.code: 	
  	(when (member :rolling mode)
  	  
  	  (setf (surface-parameters-rho params)
  		(* (surface-rho this) (surface-rho that)))
  	  
  	  (setf (surface-parameters-rho2 params)
  		(* (surface-rho2 this) (surface-rho2 that)))
  	  
  	  (setf (surface-parameters-rhoN params)
  		(* (surface-rhoN this) (surface-rhoN that)))))
      params)
  
  
negative.code: (in-package :cl-ode)
  
  (defclass proto-geometry () 
    ((surface-mode :initform '(:bounce :soft-CFM :rolling)
  		 :initarg :mode
  		 :accessor surface-mode
  		 :type :int)
     (surface-mu :INITFORM (infinity) :INITARG :mu :ACCESSOR surface-mu)
     (surface-mu2 :INITFORM 0 :INITARG :mu2 :ACCESSOR surface-mu2)
     (surface-rho :INITFORM .1d0 :INITARG :rho :ACCESSOR surface-rho)
     (surface-rho2 :INITFORM 0 :INITARG :rho2 :ACCESSOR surface-rho2)
     (surface-rhon :INITFORM 0 :INITARG :rhon :ACCESSOR surface-rhon)
     (surface-bounce :INITFORM .9 :INITARG :bounce :ACCESSOR surface-bounce)
     (surface-bounce-vel :INITFORM .01 :INITARG :bounce-vel :ACCESSOR
  		       surface-bounce-vel)
     (surface-soft-erp :INITFORM 0 :INITARG :soft-erp :ACCESSOR
  