(defun increment-alist (tbl key)
   (cond ((endp tbl) (list (cons key 1)))
         ((eql (car (first tbl)) key)
          (cons (cons key (1+ (cdr (first tbl))))
                (rest tbl)))
         (t (cons (first tbl)
                  (increment-alist (rest tbl) key)))))

(defun freq-table (xs)
   (if (endp xs)
       nil
       (increment-alist (freq-table (rest xs))
                        (first xs))))

(defun letter-freq (str)
   (freq-table (coerce str 'list)))