;;; Grammar used:
;;; 
;;; id --> letter, rest
;;;
;;; rest --> ""
;;; rest --> dash, char, rest
;;;
;;; dash --> ""
;;; dash --> "-"
;;;
;;; char --> letter
;;; char --> digit
;;; 
;;; 'letter' and 'digit' nonterminals are implemented by
;;; Lisp predicates alpha-char-p and digit-char-p, respectively.

(defun identifier (str)
  (id str ())
  )

(defun id (str stack)
  "True when STR has a prefix that is an identifier and the rest
   of STR can be parsewd sucessfully with the predicates in STACK."
  (check str (cons #'letra (cons #'resto stack)))
  )

(defun check (str stack)
  (if (null stack)
      (string= str "")
    (funcall (car stack) str (cdr stack))
    )
  )

(defun letra (str stack)
  (and
   (> (length str) 0)
   (alpha-char-p (elt str 0))
   (check (subseq str 1) stack)
   )
  )

(defun digito (str stack)
  (and
   (> (length str) 0)
   (digit-char-p (elt str 0))
   (check (subseq str 1) stack)
   )
  )

(defun resto (str stack)
  (or
   (check str stack)
   (check str (cons #'traco (cons #'carac (cons #'resto stack))))
   )
  )

(defun traco (str stack)
  (or
   (check str stack)
   (and
    (> (length str) 0)
    (string= (elt str 0) "-")
    (check (subseq str 1) stack)
    )
   )
  )

(defun carac (str stack)
  (or
   (letra str stack)
   (digito str stack)
   )
  )