var Netscape, MSIE, Opera, Unknown;
  var Win, Mac, Other;
  var NetscapeVer, MSIEVer, OperaVer;

  Netscape = navigator.appName == "Netscape";
  MSIE = navigator.appName == "Microsoft Internet Explorer";
  Opera = navigator.userAgent.indexOf("Opera") > -1;
  Unknown = !(Netscape || MSIE || Opera);

  Win = navigator.userAgent.indexOf("Win") > -1;
  Mac = navigator.userAgent.indexOf("Mac") > -1;
  Other = !(Win || Mac);

  if(Netscape) {
    NetscapeVer = parseFloat(navigator.appVersion);
  }
  else if(MSIE) {
    n = navigator.userAgent;
    MSIEVer = n.substr(n.indexOf("MSIE ")+("MSIE ").length, 4);
    MSIEVer = parseFloat(MSIEVer);

    if(Opera) {
      OperaVer = n.substr(n.indexOf("Opera ")+("Opera ").length, 4);
      OperaVer = parseFloat(OperaVer);
    }
  }

function bookmark() {
  if(Opera) {
    alert("Press Ctrl+T to bookmark this site!");
  }
  else if(Win && MSIE && MSIEVer >= 5) {
    window.external.AddFavorite(
      'http://www.iota-six.co.uk',
      'Eddie\'s Basic Guide to C, HTML + JavaScript');
  }
  else {
    alert("Press Ctrl+D to bookmark this site!");
    alert("And you should update your browser!!");
  }
}