(defun longest-common-substring (a b)
 "Return the longest substring common to a and b"
 ;; Found at https://en.wikibooks.org/wiki/Algorithm_Implementation/Strings/Longest_common_substring#Common_Lisp
  (let ((L (make-array (list (length a) (length b)) :initial-element 0))
        (z 0)
        (result '()) )
    (dotimes (i (length a))
      (dotimes (j (length b))
        (when (char= (char a i) (char b j))
          (setf (aref L i j)
                (if (or (zerop i) (zerop j))
                    1
                    (1+ (aref L (1- i) (1- j))) ))
          (when (> (aref L i j) z)
            (setf z (aref L i j)
                  result '() ))
          (when (= (aref L i j) z)
            (pushnew (subseq a (1+ (- i z)) (1+ i))
                     result :test #'equal )))))
    result ))