anchor.diff:   querySelector("#next").onClick.listen(nbutton);
   }
   void lbutton(Event e){
  -  if(num==0) {alert('Ѿǰҳ');}
  +  if(num==1) {alert('Ѿǰҳ');}
     else{
       num=num-4;
       picture1.src="club_picture[num]";
  
positive.code: 
  }
  void nbutton(Event e) {
    int n;
    if (num <= n - 4) {
      num = num + 4;
      picture1.src="picture/picture1.jpg";
    }
    else {
      alert('Ѿһҳ');
  
negative.code: var num=0;
  List<String> club_picture=[];
  void main(){
   /*for(int i=1;i<5;i++)
   {club_picture[i]='club_picture/picture'+i.toString();
   }*/
  picture1=querySelector("#picture1");
  