anchor.diff:  				   final-xn final-yn
   				   initial-index final-index)
   		 (if closed
  -		     (values (svref coord-seq (- len 2))
  -			     (svref coord-seq (- len 1))
  +		     (values (elt coord-seq (- len 2))
  +			     (elt coord-seq (- len 1))
   			     x1 y1
   			     0 (- len 2))
   		     (values x1 y1
  -			     (svref coord-seq (- len 2))
  -			     (svref coord-seq (- len 1))
  +			     (elt coord-seq (- len 2))
  +			     (elt coord-seq (- len 1))
   			     2 (- len 4)))
   	       (ecase (line-style-joint-shape line-style)
   		 (:miter
  
positive.code: 			for i from initial-index to final-index by 2
  			for xp = initial-xp then x
  			for yp = initial-yp then y
  			for x = (svref coord-seq i)
  			for y = (svref coord-seq (1+ i))
  			do (setf (values xn yn)
  				 (if (eql i final-index)
  				     (values final-xn final-yn)
  				     (values (svref coord-seq (+ i 2))
  					     (svref coord-seq (+ i
  								 3)))))
  			   (multiple-value-bind (ex1 ey1)
  			       (normalize-coords (- x xp) (- y yp))
  
negative.code: 			for i from initial-index to final-index by 2
  			for xp = initial-xp then x
  			for yp = initial-yp then y
  			for x = (elt coord-seq i)
  			for y = (elt coord-seq (1+ i))
  			do (setf (values xn yn)
  				 (if (eql i final-index)
  				     (values final-xn final-yn)
  				     (values (elt coord-seq (+ i 2))
  					     (elt coord-seq (+ i 3)))))
  			   (multiple-value-bind (ex1 ey1)
  			       (normalize-coords (- x xp) (- y yp))
  			     (multiple-value-bind (ex2 ey2)
  