(define value
  (lambda (lst)
    (if (null? lst)
        0
        (max (left-value lst)
             (right-value lst)))))

(define choose-move
  (lambda (lst)
    (let ((lv (left-value lst))
          (rv (right-value lst)))
      (cond ((> lv rv) 'left)
            ((> rv lv) 'right)
            (else 'either)))))

(define left-value
  (lambda (lst)
    (- (car lst)
       (value (cdr lst)))))

(define right-value
  (lambda (lst)
    (left-value (reverse lst))))