document.getElementById("cat0").style.display = "block";
function display(num)
 {
   for(i=0; i<27; i++)
	 {
		 catid="cat"+i;
	   if(i!=num)
       document.getElementById(catid).style.display = "none";
	   if(i==num)
       document.getElementById(catid).style.display = "block";
	   
   	 }

 }