Exercise 3.18.  Write a procedure that examines a list and determines 
whether it contains a cycle, that is, whether a program that tried to 
find the end of the list by taking successive cdrs would go into an 
infinite loop.  Exercise 3.13 constructed such lists.

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(define (cyclical? x)
  (define (go x seen)
    (cond ((memq x seen) true)
          ((null? x) false)
          (else (go (cdr x) (cons x seen)))))
  (go x '()))