function openWindow(url, height, width) {
                                        venster = window.open(url, null, 'left='+(screen.width-width) / 2 +', top='+(screen.height-height) / 2 +', height='+height+', width='+width+', resizable=yes, status=yes, toolbar=no, menubar=no, location=no, scrollbars=yes');
                                        venster.focus();
                                        return venster;
                                }

                                function _highlight(element) {
                                        return true;
                                        element.style.backgroundColor='white';
                                        element.style.color='black';
                                        return true;
                                }

                                function _normalize(element) {
                                        return true;
                                        element.style.backgroundColor='transparent';
                                        element.style.color='white';
                                        return true;
                                }

                                function _go(url) {
                                        document.location.href=url;
                                        return true;
                                }

                function showimage(filename) {
                    document.getElementById('mainImage').src = filename;
                    window.location.hash = 'imagestart';
                }
                                function check_form(required) {
                                        var errStr = '';
                                        for (i = 0; i < required.length; i++) {
                                                if (document.getElementById(required[i])) {
                                                        if (document.getElementById(required[i]).value == '') {
                                                                errStr += required[i] + ' is niet ingevuld.\n';
                                                        }
                                                }
                                        }
                                        if (errStr == '')
                                                return true;
                                        else
                                                alert(errStr + '\nUw aanvraag is niet verzonden');

                                        return false;
                                }