function show(cat) {
  var el = document.getElementById('cat_' + cat);
  z = el.style.zIndex;
  for (var i = 0; i < 3; i++) {
    el = document.getElementById('cat_' + i);
    if (el.style.zIndex > z) {
      el.style.zIndex = el.style.zIndex - 1;
    }
    else if (el.style.zIndex == z) {
      el.style.zIndex = 3;
    }
  }
}