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

        // Visible elements.
        $options = array();
        $options['lock'] = get_string('locksubmissions', 'assign');
        $options['unlock'] = get_string('unlocksubmissions', 'assign');
        $options['downloadselected'] = get_string('downloadselectedsubmissions', 'assign');
        if ($instance['submissiondrafts']) {
            $options['reverttodraft'] = get_string('reverttodraft', 'assign');
        }
        if (has_capability('mod/assign:grade', $instance['context'])) {
            $options['removesubmission'] = get_string('removesubmission', 'assign');
        }
        if ($instance['duedate'] && has_capability('mod/assign:grantextension', $instance['context'])) {
            $options['grantextension'] = get_string('grantextension', 'assign');
        }
        if ($instance['attemptreopenmethod'] == ASSIGN_ATTEMPT_REOPEN_METHOD_MANUAL) {
            $options['addattempt'] = get_string('addattempt', 'assign');
        }

        foreach ($instance['feedbackplugins'] as $plugin) {
            if ($plugin->is_visible() && $plugin->is_enabled()) {
                foreach ($plugin->get_grading_batch_operations() as $action => $description) {
                    $operationkey = 'plugingradingbatchoperation_' . $plugin->get_type() . '_' . $action;
                    $options[$operationkey] = $description;
                }
            }
        }
        if ($instance['markingworkflow']) {
            $options['setmarkingworkflowstate'] = get_string('setmarkingworkflowstate', 'assign');
        }
        if ($instance['markingallocation']) {
            $options['setmarkingallocation'] = get_string('setmarkingallocation', 'assign');
        }

        $mform->addElement('hidden', 'action', 'gradingbatchoperation');
        $mform->setType('action', PARAM_ALPHA);
        $mform->addElement('hidden', 'id', $instance['cm']);
        $mform->setType('id', PARAM_INT);
        $mform->addElement('hidden', 'selectedusers', '', array('class'=>'selectedusers'));
        $mform->setType('selectedusers', PARAM_SEQUENCE);
        $mform->addElement('hidden', 'returnaction', 'grading');
        $mform->setType('returnaction', PARAM_ALPHA);

        $objs = array();
        $objs[] =& $mform->createElement('select', 'operation', get_string('chooseoperation', 'assign'), $options);
        $objs[] =& $mform->createElement('submit', 'submit', get_string('go'));
        $batchdescription = get_string('batchoperationsdescription', 'assign');
        $mform->addElement('group', 'actionsgrp', $batchdescription, $objs, ' ', false);
    }