anchor.diff:  			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))
  +			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 (svref coord-seq (+ i 2))
  -					     (svref coord-seq (+ i 3)))))
  +				     (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)
  
positive.code: 				 (maxf max-y (+ y ny))))))))
  	       (unless closed
  		 (multiple-value-bind (x y)
  		     (values (svref coord-seq (- len 2))
  			     (svref coord-seq (- len 1)))
  		   (minf min-x (- x border))
  		   (minf min-y (- y border))
  		   (maxf max-x (+ x border))
  
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))
  