  The first three lines of this file were inserted by  . They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname ex013) (read-case-sensitive #t) (teachpacks ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp")) #f)))
(define (string-first str)
  (if (not (= (string-length str) 0))
      (substring str 0 1)
      #f))
 about the language level of this file in a form that our tools can easily process.
  The first three lines of this file were inserted by  . They record metadata
#reader(lib "htdp-beginner-reader.ss" "lang")((modname ex013) (read-case-sensitive #t) (teachpacks ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp")) #f)))
(define (string-first str)
  (if (not (= (string-length str) 0))
      (substring str 0 1)
      #f))