docs: def get_swagger_versions(settings):
      """
      Validates and returns the versions of the Swagger Spec that this pyramid
      application supports.
  
      :type settings: dict
      :return: list of strings. eg ['1.2', '2.0']
      :raises: ValueError when an unsupported Swagger version is encountered.
      """
      swagger_versions = set(aslist(settings.get(
          'pyramid_swagger.swagger_versions', DEFAULT_SWAGGER_VERSIONS)))
  
      if len(swagger_versions) == 0:
          raise ValueError('pyramid_swagger.swagger_versions is empty')
  
      for swagger_version in swagger_versions:
          if swagger_version not in SUPPORTED_SWAGGER_VERSIONS:
              raise ValueError('Swagger version {0} is not supported.'
                               .format(swagger_version))
      return swagger_versions