Exercise 3.1. An accumulator is a procedure that is called repeatedly with a single numeric argument and accumulates its arguments into a sum. Each time it is called, it returns the currently accumulated sum. Write a procedure make-accumulator that generates accumulators, each maintaining an independent sum. The input to make-accumulator should specify the initial value of the sum; for example
(define A (make-accumulator 5))
(A 10)
15
(A 10)
25
函数的参数可以当做内部变量来使用，所以 ``make-accumulator`` 定义如下：
(define (make-accumulator sum)
(lambda (add)
(set! sum (+ add sum))
sum))