anchor.diff:    (tail nil :type cons))
   
   (defun frontier-fifo ()
  -  (let1 cell (new-cons)
  +  (let1 cell (cons^)
       (%make-frontier-fifo :head cell :tail cell)))
   
   
  
positive.code:         (let1 f (first cell)
            (setf (frontier-fifo-head fifo) (rest cell)
                  (first cell) nil)
            (cons-to-pool cell)
            (the frontier f)))))
  
  (defun clear-frontier-fifo (fifo)
  
negative.code:   nil)
  
  
  (defun free-frontier-fifo (head tail)
    "Add a list of FRONTIERs to the free frontier pool.
  TAIL must be the last CONS cell of the list starting at HEAD."
    (declare (type cons head tail))
  