code: def by_content_type():
      
      def f(msg):
          content_type = glance(msg, flavor='chat')[0]
          return content_type, (msg[content_type],)
      return f
docs: :return:
          A key function that returns a 2-tuple (content_type, (msg[content_type],)).
          In plain English, it returns the message's *content type* as the key,
          and the corresponding content as a positional argument to the handler
          function.