练习1.3

(define (smaller-or-equal x y) (or (< x y) (= x y)))

(define (min-of-three a b c) 
    (cond ((and (smaller-or-equal a b) (smaller-or-equal a c)) a)
          ((and (smaller-or-equal b a) (smaller-or-equal b c)) b)
          (else c)))

(define (sum a b c) (- (+ a b c) (min-of-three a b c)))