docs: def serialize_comment(ctx, document, el, root):
      "Serializes comment."
  
      # Check if option is turned on
  
      if el.comment_type == 'end':
          ctx.opened_comments.remove(el.cid)
      else:
          if el.comment_type != 'reference':
              ctx.opened_comments.append(el.cid)
  
          if ctx.options['comment_span']:
              link = etree.SubElement(root, 'a')
              link.set('href', '#')
              link.set('class', 'comment-link')    
              link.set('id', 'comment-id-' + el.cid)    
  
              link.text = ''
  
              fire_hooks(ctx, document, el, link, ctx.get_hook('comment'))
  
      return root