var snum = 0;
var smax = 13;
var undef;
function go(inc) {
  var cid = "s"+snum;
  snum+=inc;
  if (snum==smax+1) snum=0;
  if (snum<0) snum=smax;
  var nid = "s"+snum;
  var ne = document.getElementById(nid);
  if (ne==undef) {
    ne = document.getElementById("s0");
    snum = 0;
  }
  document.getElementById(cid).style.visibility = "hidden";  
  ne.style.visibility = "visible";
}