function popup(url,a,b,c,d) {
	var breedte = a ? a : 350;
	var hoogte = b ? b : 180;
	var features = c ? c : '';
	var windowName = d ? d : 'Help';

	var boven = 100;
	var links = 100;
	if (document.all || document.layers) {
		boven = Math.floor((top.screen.height - hoogte)/3);
		links = Math.floor((top.screen.width - breedte)/2);
	}
	var featurestring = 'scrollbars=1,width='+breedte + ',height='+hoogte + ',screenX='+links+ ',screenY='+boven +',left='+links+ ',top='+boven;
	if (features != '') featurestring += ','+ features;

	var popup = window.open(url,windowName,featurestring);
	popup.opener = self;
	popup.focus();
}