public function definition() {
        $mform = $this->_form;

        $contexts = $this->_customdata['contexts'];
        $usablecontexts = $contexts->having_cap('moodle/question:useall');

        // Standard fields at the start of the form.
        $mform->addElement('header', 'generalheader', get_string("general", 'form'));

        $mform->addElement('questioncategory', 'category', get_string('category', 'question'),
                array('contexts' => $usablecontexts, 'top' => true));

        $mform->addElement('advcheckbox', 'includesubcategories', get_string('recurse', 'quiz'), null, null, array(0, 1));

        $tops = question_get_top_categories_for_contexts(array_column($contexts->all(), 'id'));
        $mform->hideIf('includesubcategories', 'category', 'in', $tops);

        $tags = \core_tag_tag::get_tags_by_area_in_contexts('core_question', 'question', $usablecontexts);
        $tagstrings = array();
        foreach ($tags as $tag) {
            $tagstrings["{$tag->id},{$tag->name}"] = $tag->name;
        }
        $options = array(
                'multiple' => true,
                'noselectionstring' => get_string('anytags', 'quiz'),
        );
        $mform->addElement('autocomplete', 'fromtags', get_string('randomquestiontags', 'mod_quiz'), $tagstrings, $options);
        $mform->addHelpButton('fromtags', 'randomquestiontags', 'mod_quiz');

        $mform->addElement('hidden', 'slotid');
        $mform->setType('slotid', PARAM_INT);

        $mform->addElement('hidden', 'returnurl');
        $mform->setType('returnurl', PARAM_LOCALURL);

        $buttonarray = array();
        $buttonarray[] = $mform->createElement('submit', 'submitbutton', get_string('savechanges'));
        $buttonarray[] = $mform->createElement('cancel');
        $mform->addGroup($buttonarray, 'buttonar', '', array(' '), false);
        $mform->closeHeaderBefore('buttonar');
    }