function showSubMenu (submenu) {
  if(document.getElementById) {
    document.getElementById("submenu_wer").style.visibility = "hidden";
    document.getElementById("submenu_was").style.visibility = "hidden";
    document.getElementById("submenu_wie").style.visibility = "hidden";

    document.getElementById("submenu_" + submenu).style.visibility = "visible";
  }
} 

function showPage (page) {
  if(document.getElementById) {
    document.getElementById("firma").style.visibility = "hidden";
    document.getElementById("mitarbeiter").style.visibility = "hidden";
    document.getElementById("partner").style.visibility = "hidden";

    document.getElementById("webdesign").style.visibility = "hidden";
    document.getElementById("programmierung").style.visibility = "hidden";
    document.getElementById("datenbank").style.visibility = "hidden";
    document.getElementById("contentmanagementsystem").style.visibility = "hidden";
    document.getElementById("optimierung").style.visibility = "hidden";
    document.getElementById("eintragung").style.visibility = "hidden";
    document.getElementById("internationalisierung").style.visibility = "hidden";

    document.getElementById("contact").style.visibility = "hidden";
    document.getElementById("mail").style.visibility = "hidden";
    document.getElementById("impressum").style.visibility = "hidden";

    document.getElementById(page).style.visibility = "visible";
  }
}