anchor.diff:      (unless (eq head tail)
         (rotatef (first head) (first tail))
         (let1 next (rest head)
  -        (frontier-fifo-to-pool next tail))
  +        (free-frontier-fifo next tail))
         (setf (rest head) nil
               (frontier-fifo-tail fifo) head))
       fifo))
  
positive.code:   nil)
  
  
  (defun frontier-to-pool (f)
    "Add frontier F to the free frontier pool."
    (declare (type frontier f))
    (cons-and-frontier-to-pool (new-cons f)))
  
  
  
  
negative.code:   "Create and append a new FRONTIER to FIFO."
    (declare (type frontier-fifo fifo)
             (type fixnum x y z dw))
    (let* ((cell (new-cons-and-frontier))
           (f    (first cell)))
  
      (if f
          (setf (first    cell) nil
                (frontier-x  f) x
                (frontier-y  f) y
                (frontier-z  f) z
                (frontier-dw f) dw)
          (setf f (frontier x y z dw)))
  
      (let1 tail (frontier-fifo-tail fifo)
        (setf (first tail) f
  