var curr=0;
oldLayerName="layer"+curr;
function next(){
   if(curr<max-1){
      oldLayerName="layer"+curr;
      curr++;
      showLayer("layer"+curr,"500");
   }
}
function prev(){
   if(curr>0){
      oldLayerName="layer"+curr;
      curr--;
      showLayer("layer"+curr,"500");
   }
}