Drupal formalter for a separate teaser field

/**
 * Implementation of hook_form_alter().
 */
function separate_teaser_form_alter($form_id, &$form) {
  if (isset($form['type']) && ($form['type']['#value'] .'_node_form' == $form_id)) {
    $form['teaser'] = array(
      '#type' => 'textarea',
      '#title' => t('Teaser'),
      '#description' => t('The teaser is used as short introduction of this post. It is displayed in all sorts of listings. If this is left empty, the system will create a teaser automatically.'),
      '#weight' => $weight,
      '#default_value' => $node->teaser,
      '#required' => $required,
    );
  }
}