Exercise 2.18.  Define a procedure reverse that takes a list as argument 
and returns a list of the same elements in reverse order:

(reverse (list 1 4 9 16 25))
(25 16 9 4 1)

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(define (reverse items)
  (define (go in out)
    (if (null? in)
        out
        (go (cdr in) (cons (car in) out))))
  (go items nil))