Exercise 3.14: The following procedure is quite
useful, although obscure:


(define (mystery x)
  (define (loop x y)
    (if (null? x)
        y
        (let ((temp (cdr x)))
          (set-cdr! x y)
          (loop temp x))))
  (loop x '()))

Loop uses the “temporary” variable temp to hold the old value
of the cdr of x, since the set-cdr!  on the next line
destroys the cdr.  Explain what mystery does in general.  Suppose
v is defined by (define v (list 'a 'b 'c 'd)). Draw the
box-and-pointer diagram that represents the list to which v is bound.
Suppose that we now evaluate (define w (mystery v)). Draw
box-and-pointer diagrams that show the structures v and w after
evaluating this expression.  What would be printed as the values of v
and w?