var activeMenu = null;

function showMenu(menuId){  
  var menu;
  if (activeMenu != null){
    activeMenu.style.visibility="hidden";
  }
  menu = document.getElementById(menuId);
  menu.style.visibility= "visible";
  activeMenu = menu;
}

function clearMenus(){
  if (activeMenu != null){
    activeMenu.style.visibility="hidden";
  }
  activeMenu = null;
}