求三个数中较大的两个数之和
(define (sum-of-two x y) (+ x y) )
(define (largest-two-sum x y z)
(cond
( (= (min x y z) x) (sum-of-two y z) )
( (= (min x y z) y) (sum-of-two x z) )
( (= (min x y z) z) (sum-of-two x y) ) ) )