(defmacro palinfun (name parameters &body body)
  `(defun ,name (sequence ,@parameters)
     (let* ((len (length sequence)) (anti-midlen (antibelt sequence (floor len 2))) (midlen (floor len 2)) (midsequence (/ len 2)))
       (declare (ignorable midlen anti-midlen midsequence))
       ,@body)))

(palinfun lrbelt (belt)
  ;;Returns 0 (left) 1 (right) or 2 (mid) for position of belt in sequence
  (when (> belt (1- len)) (return-from lrbelt nil))
  (when (and (oddp len) (= belt midlen)) (return-from lrbelt 2))
  (when (and (evenp len) (= belt midlen) (return-from lrbelt 1)))
  (when (< belt midsequence) (return-from lrbelt 0))
  (when (> belt midsequence) (return-from lrbelt 1)))

(palinfun antibelt (belt)
;;;Returns the corresponding belt across the midline of sequence
  (let ((zblen (- len 1)))
    (- zblen belt)))

(palinfun belt (elt)
;;;A balanced version of elt
(when (> elt (- len 1)) (return-from belt nil))
(cons (elt sequence elt) (elt (reverse sequence) elt)))


(palinfun bset (belt newval)
;;;Outputs a copy of sequence with belt and antibelt set to newval
(let ((leftbelt (if (zerop (lrbelt sequence belt)) (values 1) (values nil)))
      (midbelt (if (= (lrbelt sequence belt) 2) (values 1) (values nil))) (antibelt (antibelt sequence belt)))
  (when (> belt (1- len)) (return-from bset nil))
  (let ((rightbelt (if (and (not leftbelt) (not midbelt)) (values 1) (values nil))))
    (cond (midbelt
       (concatenate 'string (subseq sequence 0 belt) newval (subseq sequence (1+ belt) len)))
      (leftbelt
       (concatenate 'string (subseq sequence 0 belt) newval (subseq sequence (1+ belt) antibelt) newval (subseq sequence (1+ antibelt) len)))
      (rightbelt
       (concatenate 'string (subseq sequence 0 antibelt) newval (subseq sequence (1+ antibelt) belt) newval (subseq sequence (1+ belt) len)))))))

(palinfun palindromep ()
;;;Palindrome predicate
  (loop for n from 0 upto anti-midlen do
    (let ((belt (belt sequence n)))
      (let ((left (car belt)) (right (cdr belt)))
    (when (not (equal left right)) (return-from palindromep nil))))
    finally (return-from palindromep 1)))

(palinfun palindrominc ()
;;;Increments palindrome to next palindrome
      (when (not (equal 9 (parse-integer (string (elt sequence anti-midlen))))) (return-from palindrominc (bset sequence anti-midlen (write-to-string (1+ (parse-integer (string (elt sequence anti-midlen))))))))
      (loop for belt from (1- anti-midlen) downto 0 do
    (when (not (equal 9 (parse-integer (string (elt sequence belt))))) (return-from palindrominc (bset sequence belt (write-to-string (1+ (parse-integer (string (elt sequence belt))))))))
        finally (return-from palindrominc (write-to-string (1+ (expt 10 len))))))

(palinfun right-balance ()
;;;Balance right digits with left digits
  (loop for belt from (1- len) downto midlen with newsequence = sequence do
    (when (< (parse-integer (string (elt newsequence belt))) (parse-integer (string (elt newsequence (antibelt newsequence belt)))))
      (setf newsequence (bset newsequence belt (string (elt newsequence (antibelt newsequence belt))))))
    finally (return newsequence)))

(palinfun next-palindrome ()
;;;Balance left digits with right digits
      (loop for belt from anti-midlen downto 0 with newsequence = sequence do
    (when (< (parse-integer (string (elt newsequence belt))) (parse-integer (string (elt newsequence (antibelt newsequence belt)))))
      (setf newsequence (bset newsequence belt (string (elt newsequence (antibelt newsequence belt))))))
    finally (return newsequence)))