function switchBlock(element, positie){
	var retVal = new Array();
	var elements = document.getElementsByClassName("tab_active");
	for(var i = 0;i < elements.length;i++){
		classelement = elements[i];
		
		factor = positie - 1;
		opacity_value = factor * 10;
		
		opacity = 100 - opacity_value;
		
		setOpacity(opacity, classelement);
		classelement.className = 'tab';
	}
	element.className = 'tab_active';
}

function setOpacity(value, element) {
	var opacity	= value / 10;

	element.style.opacity = opacity;
	/*element.style.filter = 'alpha(opacity=' + value * 10 + ')';*/
}