var currentMenu;

function showMenu(menu)
{
   hideMenu();
   window.document.getElementById(menu).style.visibility = 'visible';
   currentMenu = menu;
}

function hideMenu()
{
   if(currentMenu != null)
   {
      window.document.getElementById(currentMenu).style.visibility = 'hidden';
      currentMenu = null;
   }
}

function changeMenu(menu)
{
   if(currentMenu != menu)
   {
      showMenu(menu);
   }
   else
   {
      hideMenu();
   }
}