docs: def _define(self, key, default=_UNSET, help=_UNSET, validate=_UNSET):
          """ (Unstable) Shortcut for plugins to define own config parameters. """
          if default is not _UNSET:
              self.setdefault(key, default)
          if help is not _UNSET:
              self.meta_set(key, 'help', help)
          if validate is not _UNSET:
              self.meta_set(key, 'validate', validate)