racket - BNF 文法を s 式に変換する

Question

という文法を定義しましたmsl

#lang plai-typed
(define-type msl
  [msl-num (n : number)]
  [msl-add (l : msl) (r : msl)]
  [msl-mul (l : msl) (r : msl)]
  [msl-sub (l : msl) (r : msl)]
  [msl-pow (l : msl) (r : msl)]
  [msl-error (s : string)]
  )

(define(** u t)
       (cond
         ((= u 1) t)
         (else
          (* t(**(sub1 u) t)))))

に変換するパーサー関数がありs-expressionますmsl

(define (parse [s : s-expression]) : msl
  (cond
    [(s-exp-number? s) (msl-num (s-exp->number s))]
    [(s-exp-list? s)
     (let ([sl (s-exp->list s)])
       (case (s-exp->symbol (first sl))
         [(+) (msl-add (parse (second sl)) (parse (third sl)))]
         [(*) (msl-mul (parse (second sl)) (parse (third sl)))]
         [ (-) (msl-sub (parse (second sl)) (parse (third sl)))]
         [ (**) (msl-pow (parse (second sl)) (parse (third sl))) ]
         [else (error 'parse "invalid list input")]))]
    [else (error 'parse "invalid input")]))

msl私の質問は次のとおりです。式をこのように変換するにはどうすればよいs-expressionですか? 私はこのテーマの初心者です