The inverse function inv

We need to be able to calculate the inverse of a given generator. So the inverse of 'a' is 'A', 'A' is 'a', 'b' is 'B' and 'B' is 'b'. This function, given a string composed of generators rewrites the string by reversing the string and doing a generator by generator replacement.

(defun inv (word)
 "given a word, construct its inverse"
 (let ((result (reverse word)))
  (dotimes (i (length result))
   (setf (elt result i)
    (cond
     ((eq (elt result i) #\\a) #\\A)
     ((eq (elt result i) #\\b) #\\B)
     ((eq (elt result i) #\\A) #\\a)
     ((eq (elt result i) #\\B) #\\b))))
  result))