An example of parsing a simple sexp in instaparse

instaparse-sexp.clj

(ns parse.core
  (:require [instaparse.core :as insta]))

(def parser
    (insta/parser
        "sexp = lparen operation rparen
         <lparen> = <'('>
         <rparen> = <')'>
         operation = operator + args
         operator = '+'
         args = snumber+ ssexp* 
         <ssexp>   = space sexp
         <snumber> = space number
         <space>  = <#'[ ]*'> 
         number = #'[0-9]+'
        "))

(defn choose-op [op]
  (case op
    "+" +))
(def transform-options
  {:number read-string
   :args vector
   :operator choose-op
   :operation apply
   :sexp identity
   })

(defn lisp [input]
  (->> (parser input) (insta/transform transform-options)))

(lisp "(+ 1 2 (+ 3 4))") ;; => 10

Anyone can tell me why we put a "+" in line9? Thanks.