function definition() {
        $this->caller = $this->_customdata['caller'];
        $options = $this->_customdata['options'];
        $mform =& $this->_form;
        $mform->addElement('select', 'instance', get_string('selectplugin', 'portfolio'), $options);
        $mform->addElement('hidden', 'id', $this->_customdata['id']);
        $mform->setType('id', PARAM_INT);
        $this->add_action_buttons(true, get_string('next'));
    }