docs: def standard_licenses_only(build_context, target) -> str:
      """A policy function for allowing specifying only known licenses.
  
      Return error message (string) if policy for `target` is violated,
      otherwise return `None`.
  
      To apply in project, include this function in the ilst returned by the
      `get_policies` function implemented in the project `YSettings` file.
  
      See example in tests/errors.
      """
      for license_name in target.props.license:
          if license_name not in KNOWN_LICENSES:
              # TODO: include suggestion for similar known license
              return 'Unknown license: {}'.format(license_name)
      return None