function koersen_switchTo(koers1, koers2, koers3, koers4)
 	{
		//activate selected tab and show image
		document.getElementById(koers1).className = "selected";
		document.getElementById('img_'+koers1).className = "selected";
		document.getElementById(koers2).className = "";
		document.getElementById('img_'+koers2).className = "hidden";
		document.getElementById(koers3).className = "";
		document.getElementById('img_'+koers3).className = "hidden";
		document.getElementById(koers4).className = "";
		document.getElementById('img_'+koers4).className = "hidden";
 	}