"Return the function that finds the difference from num."

#'(lambda (x) (abs (- x num))))

(defun sorter (cost-fn)

"Return a combiner function t h a t sorts according to cost-fn."

#'(lambda (new old)

(sort (append new old) #'< :key cost-fn)))

(defun best-first-search ( s t a r t goal-p successors cost-fn)