function openCentered (url, width, height, windowName, featureString) {
  if (!windowName)
    windowName = '';
  if (!featureString)
    featureString = '';
  else
    featureString = ',' + featureString;
  var x = Math.round((screen.availWidth - width) / 2);
  var y = Math.round((screen.availHeight - height) / 2);
  featureString = 'left=' + x + ',top=' + y + ',width=' + width 
+ ',height=' + height + featureString;
  open (url, windowName, featureString);
}