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homework.lisp

(defmodule homework (export letters))

(defun lut (x) 
  (: lists nth (- x (car '"/")) 
              '("0" 
                "1"    "abc" "def" 
                "ghi"  "jkl" "mno" 
                "pqrs" "tuv" "wxyz")))

(defun letters 
  (('()) '(())) 
  (((cons n ns)) (lc ((<- chr (lut n)) 
                      (<- rest (letters ns))) 
                  (cons chr rest))))


;; Example test (no I/O)
(defun test ()
  (let* ((t1 (letters '"123"))
         (t2 (letters '"456"))
         (t3 (letters '"789"))
         (9  (length t1))
         (27 (length t2))
         (48 (length t3)))
        (tuple t1 t2 t3)))