function openSubmenu (menuNr) {
  clearSubMenu ();
  if (document.getElementById) document.getElementById ('submenu' + menuNr).style.visibility = 'visible';
}

function closeSubmenu (menuNr) {
  if (document.getElementById) document.getElementById ('submenu' + menuNr).style.visibility = 'hidden';
}

function clearSubMenu () {
  for (var i = 0; i < menuItemAmount; i ++) closeSubmenu (i);
}