generate-index.el

(defun generate-index (n &optional m l)
  (let ((base (mapcar 'char-to-string (number-sequence ?a ?z))))
    (cond
     ((null m)
      (generate-index n 0 l))
     ((<= n m)
      l)
     ((null l)
      (generate-index n (1+ m) base))
     (t
      (generate-index
       n (1+ m)
       (apply 'append l (mapcar (lambda (str)
                                  (mapcar (lambda (e)
                                            (concat str e))
                                          l))
                                base)))))))

(generate-index 3)