code: def format_author_line(author_names):
      
      author_line = None
      if not author_names:
          return author_line
      if len(author_names) <= 2:
          author_line = ", ".join(author_names)
      elif len(author_names) > 2:
          author_line = author_names[0] + " et al."
      return author_line
docs: authorLine format depends on if there is 1, 2 or more than 2 authors