(define priority-op
  { symbol --> number }
  not -> 1
  uminus -> 7)

(define pp-binop
  { symbol --> string }
  plus -> "+"
  mult -> "*")

(define parenthesis
  { string --> string }
  X -> (@s "(" X ")"))

(define isdigit
  { string --> boolean }
  C -> (let CN (string->n C)
         (and (>= CN (string->n "0")) (<= CN (string->n "9")))))