问：定义一个过程，它以三个数为参数，返回其中较大的两个数之和
解：
(define min(a b)
(if (< a b) a b)
)
(define minnum(a b c)
(min(min a b) c)
)
(define sumbig( a b c)
(
COND(= a minnum(a b c ))
(+ b c)
COND(= b minnum(a b c ))
(+ a c)
COND(= c minnum(a b c ))
(+ a b)
)
)
