var check = true;
function check_selectie(form, element){
	for (i=0; i < document.forms[form].elements.length; i++) {
		if(document.forms[form].elements[i].name == element+"[]"){
			document.forms[form].elements[i].checked = (check)? 1 : 0;
		}
	}
	check = !check;
}
	
function is_selected(form, element, foutmelding){
	var selected = 0;
	for (i=0; i < document.forms[form].elements.length; i++) {
		if(document.forms[form].elements[i].name == element+"[]" && document.forms[form].elements[i].checked == 1){
			selected++;
		}
	}
	
	if(selected > 0){
		return true;
	}
	else{
		alert(foutmelding);
		return false;
	}
}