oldLayer=""
 function showLayer(layerName) {
  if (oldLayer) {
   document.all[oldLayer].style.visibility = 'hidden';
  }
  document.all[layerName].style.visibility = 'visible';
  oldLayer=layerName;
 }
 function hideLayer(layerName) {
  document.all[layerName].style.visibility = 'hidden'; 
 }