var box_visible = false;

function show(divid) {
  if (divid == box_visible)
    return;

  box_visible = divid;

  for(a = 11; a < 19; a++) {    
    if ("box"+a != divid)
      document.getElementById("box"+a).style.visibility="hidden";
    else
      document.getElementById("box"+a).style.visibility="visible";
  }
}

function hide(divid) {
  document.getElementById(divid).style.visibility="hidden";
}