function ShowZelfdoenStep(stepno) {
	//make sure 'zelfdoen' element is centered in visible window
	var pageWidth, pageHeight, windowWidth, windowHeight, xOffs, yOffs;
	var pageSize = getPageSize();
	var windowSize = getVisibleWindowSize();
	var scrollOffs = getScrollOffsets();
	
	//page
	pageWidth = pageSize[0];
	pageHeight = pageSize[1];
	//set element to full page
	document.getElementById('zelfdoen_back').style.width = pageWidth + 'px';
	document.getElementById('zelfdoen_back').style.height = pageHeight + 'px';
	//show it
	ShowElementById('zelfdoen_back');
	//visible window
	windowWidth = windowSize[0];
	windowHeight = windowSize[1];
	//scrolloffsets
	xOffs = scrollOffs[0];
	yOffs = scrollOffs[1];
	//center element
	document.getElementById('zelfdoen').style.left = windowWidth/2-400+xOffs + 'px';
	document.getElementById('zelfdoen').style.top = windowHeight/2-300+yOffs + 'px';
	//show it
	ShowElementById('zelfdoen');
	//fill it
	AjaxCall('zelfdoen.php','show_explanationstep', 'stepno=' + stepno);
	}
	
function HideZelfdoen() {
	HideElementById('zelfdoen');
	HideElementById('zelfdoen_back');
	}