docs: def get_help_usage(command):
      # type: (str) -> None
      """Print out a help message and exit the program.
  
      Args:
          command: If a command value is supplied then print the help message for
              the command module if available. If the command is '-a' or '--all',
              then print the standard help message but with a full list of
              available commands.
  
      Raises:
          ValueError: Raised if the help message is requested for an invalid
              command or an unrecognized option is passed to help.
      """
      if not command:
          doc = get_primary_command_usage()
      elif command in ('-a', '--all'):
          subcommands = [k for k in settings.subcommands if k is not None]
          available_commands = subcommands + ['help']
          command_doc = '\nAvailable commands:\n{}\n'.format(
              '\n'.join('  {}'.format(c) for c in sorted(available_commands)))
          doc = get_primary_command_usage(command_doc)
      elif command.startswith('-'):
          raise ValueError("Unrecognized option '{}'.".format(command))
      elif command in settings.subcommands:
          subcommand = settings.subcommands[command]
          doc = format_usage(subcommand.__doc__)
      docopt.docopt(doc, argv=('--help',))