(defun is-palindrome-p (ints)
  "return T iff ints is a palindrome. Use constant space"
  (labels ((aux (xs lower higher)
             (cond ((null xs) nil)
                   ((> lower higher) t)
                   ((not (= (nth lower xs)
                            (nth higher xs)))
                    nil)
                   (t (aux xs (1+ lower) (1- higher))))))
    (aux ints 0 (1- (length ints)))))