if (document.layers){
      //Netscape 4 specific code
      pre = 'document.';
      post = '';
   }
   if (document.getElementById){
      //Netscape 6 specific code
      pre = 'document.getElementById("';
      post = '").style';
   }
   if (document.all){
      //IE4+ specific code
      pre = 'document.all.';
      post = '.style';
   }

// set currentLayer to the bottom (default) layer that is visible when menus are closed 
var currentLayer = 'one';

   function showLayer(lyr){
      hideLayer(currentLayer);
      eval(pre + lyr + post).visibility = 'visible';
      currentLayer = lyr;
   }
   function hideLayer(lyr){
      eval(pre + lyr + post).visibility = 'hidden';
   }