(define letter-to-number
  (lambda (letter)
    (if (eq? letter 'A)
        4.0
        (if (eq? letter 'B+)
            3.5
            (if (eq? letter 'B)
                3.0
                0)))))

(define letter-to-number
  (lambda (letter)
    (cond [(eq? letter 'A) 4.0] 
          [(eq? letter 'B+) 3.5]
          [(eq? letter 'B) 3.0]
          [else 0])))

(define letter-to-number
  (lambda (letter)
    (case letter
      [(A) 4.0] 
      [(B+) 3.5]
      [(B C) 3.0]
      [else 0])))

(define largest-in-list
  (lambda (lon)
    (if (null? lon)
	(errorf 'largest-in-list "list is empty")
	(largest-in-nonempty-list (car lon) (cdr lon)))))

(define largest-in-nonempty-list
  (lambda (largest-so-far unseen)
    (if (null? unseen)
	largest-so-far
	(largest-in-nonempty-list (max (car unseen)
				       largest-so-far)
	                           ; (if (> (car unseen)
					; largest-so-far)
				      ;(car unseen)
				      ;largest-so-far)
                        	  (cdr unseen)))))