/**
  * Echo comment form content.
  *
  * Called by the comment_form action.
  *
  * @param $post_id
  */
 public static function form_content($post_id)
 {
     if (!Prompt_Core::$options->get('prompt_key') or !Prompt_Core::$options->get('enable_comment_delivery')) {
         return;
     }
     self::enqueue_assets($post_id);
     self::$prompt_post = new Prompt_Post($post_id);
     $current_user = Prompt_User_Handling::current_user();
     if ($current_user and self::$prompt_post->is_subscribed($current_user->ID)) {
         return;
     }
     echo html('label id="prompt-comment-subscribe"', html('input', array('type' => 'checkbox', 'name' => self::SUBSCRIBE_CHECKBOX_NAME, 'value' => '1', 'checked' => Prompt_Core::$options->get('comment_opt_in_default'))), ' ', html('span', Prompt_Core::$options->get('comment_opt_in_text')));
 }