Fooling around with grammar definition syntaxes

grammar.rkt

(define (zero-or-more term)
  (one-of term
          (sequence-of term (zero-or-more term))))

(define nat-addition-grammar
  (grammar [NUMBER (one-of DIGIT
                           (sequence-of NONZERO-DIGIT (zero-or-more DIGIT)))]
           [DIGIT (one-of NONZERO-DIGIT
                          (literal 0))]
           [NONZERO-DIGIT (one-of (literal 1) (literal 2) (literal 3) (literal 4) (literal 5) (literal 6) (literal 7) (literal 8) (literal 9))]
           [EXPRESSION (sequence-of NUMBER (zero-or-more (sequence-of PLUS NUMBER)))]
           [PLUS (literal "+")]))

(define-syntax-rule (grammar [term-id term] ...)
  (letrec ([term-id term] ...)
    (grammar (make-immutable-hash (('term-id . term-id) ...)))))