docs: def get_primary_command_usage(message=''):
      # type: (str) -> str
      """Return the usage string for the primary command."""
      if not settings.merge_primary_command and None in settings.subcommands:
          return format_usage(settings.subcommands[None].__doc__)
      if not message:
          message = '\n{}\n'.format(settings.message) if settings.message else ''
      doc = _DEFAULT_DOC.format(message=message)
      if None in settings.subcommands:
          return _merge_doc(doc, settings.subcommands[None].__doc__)
      return format_usage(doc)