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