;; Use the first argument to decide in which direction to look first: 
(define (Next-Floor up-or-down CF alod)
  (cond
    ((eq? 'up up-or-down) (Next-Floor-Up CF alod))
    ((eq? 'down up-or-down) (Next-Floor-Down CF alod))
    (else (error 'Next-Floor "bad input"))))

;; The elevator is going up. 
;; Find out whether there are any demands for going up further. 
;; If yes, pick the smallest number. That's the next floor.
;; If no, send the elevator down. The largest number is the next floor. 
(define (Next-Floor-Up CF alod)
  (cond
    ((cons? (demands-going-up alod CF))
     (smallest (demands-going-up alod CF)))
    ((cons? alod) (largest alod))
    (else CF)))

;; The elevator is going down. 
;; Find out whether there are any demands for going down further. 
;; If yes, pick the largest number. That's the next floor.
;; If no, send the elevator up. The largest number is the next floor. 
(define (Next-Floor-Down CF alod)
  (cond
    ((cons? (demands-going-down alod CF))
     (smallest (demands-going-down alod CF)))
    ((cons? alod) (largest alod))
    (else CF)))