### INTRODUCTORY-CHALLENGE-PROBLEM-1

challenge problem 1 for the new user of ACL2
Major Section: INTRODUCTION-TO-THE-THEOREM-PROVER

Start in a fresh ACL2, either by restarting your ACL2 image from scratch or
executing

:ubt! 1

which will undo everything since the first user event.
Then define this function:

(defun rev (x)
(if (endp x)
nil
(append (rev (cdr x)) (list (car x)))))

Then use The Method to prove:
(defthm triple-rev
(equal (rev (rev (rev x))) (rev x)))

When you've solved this problem, compare your answer to ours;
see introductory-challenge-problem-1-answer.

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