gistfile1.cl

(defun add-at (a l pos)
  (if (= pos 0)
      (cons a l)
    (cons (car l) (add-at a (rest l) (1- pos)))))

(defun insert-everywhere/in-a-word (char word)
  (loop for position from 0 to (length word)
        collecting (add-at char word position)))

(defun insert-in-all (char word)
  (if (null word) 
      (list char)
   (append (list char (car word)) (insert-in-all char (rest word)))))

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CL-USER 38 > (INSERT-EVERYWHERE/IN-A-WORD 'a '(t e s t))
((A T E S T) (T A E S T) (T E A S T) (T E S A T) (T E S T A))

---

CL-USER 56 > (insert-in-all 'a '(t e s t))
(A T A E A S A T A)