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

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(define (make-accumulator n)
  (lambda (x)
    (begin (set! n (+ n x))
           n)))