getViewportScrollTop = function() {
	
	var x = 0;
	if (self.scrollTop) // all except Explorer
	{
		x = self.scrollTop;
	}
	else if (document.documentElement && document.documentElement.scrollTop)
		// Explorer 6 Strict Mode
	{
		x = document.documentElement.scrollTop;
	}
	else if (document.body) // other Explorers
	{
		x = document.body.scrollTop;
	}
	return x;		
};

/*
    Written by Jonathan Snook, http://www.snook.ca/jonathan
    Add-ons by Robert Nyman, http://www.robertnyman.com
*/

function getElementsByClassName(oElm, strTagName, strClassName){
    var arrElements = (strTagName == "*" && oElm.all)? oElm.all : oElm.getElementsByTagName(strTagName);
    var arrReturnElements = new Array();
    strClassName = strClassName.replace(/\-/g, "\\-");
    var oRegExp = new RegExp("(^|\\s)" + strClassName + "(\\s|$)");
    var oElement;
    for(var i=0; i<arrElements.length; i++){
        oElement = arrElements[i];      
        if(oRegExp.test(oElement.className)){
            arrReturnElements.push(oElement);
        }   
    }
    return (arrReturnElements)
}


var hideTopLinksIntervalId = -1; 
var firstTime = true;

function hideTopLinks() {
				
	var vscroll = getViewportScrollTop() > 0;
	var displayType;
		
	if(vscroll) {
		displayType = "";
		if (hideTopLinksIntervalId != -1) clearInterval(hideTopLinksIntervalId);
	} else {
		displayType	= "none";
	}
	
	var allLinks = getElementsByClassName(document, "a", "link-naar-boven");
	var oElement;
	for(var i=0; i<allLinks.length; i++){
		oElement = allLinks[i]; 
		oElement.style.display = displayType; 
	}

	if (firstTime) {
		hideTopLinksIntervalId = setInterval("hideTopLinks()", 500);
		firstTime = false;
	} 
	
}