(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 "empty list ~s" (+ 6 7))
	(largest-in-nonempty (car lon) (cdr lon)))))

(define largest-in-nonempty
  (lambda (largest-so-far not-yet-seen)
    (if (null? not-yet-seen)
	largest-so-far
	(largest-in-nonempty (max largest-so-far (car not-yet-seen))
			     (cdr not-yet-seen)))))






(vector-sum '(1 2 3) '(4 5 6)) ; (5 7 9)






(define vector-sum
  (lambda (u v)
    (list (+ (car u) (car v))
	  (+ (cadr u) (cadr v))
	  (+ (caddr u) (caddr v)))))