code: def _check_enum(parameter_name, value, parameter_config):
    
    enum_values = [enum['backendValue']
                   for enum in parameter_config['enum'].values()
                   if 'backendValue' in enum]
    if value not in enum_values:
      raise errors.EnumRejectionError(parameter_name, value, enum_values)
docs: Checks if an enum value is valid.
  
    This is called by the transform_parameter_value function and shouldn't be
    called directly.
  
    This verifies that the value of an enum parameter is valid.
  
    Args:
      parameter_name: A string containing the name of the parameter, which is
        either just a variable name or the name with the index appended. For
        example 'var' or 'var[2]'.
      value: A string containing the value passed in for the parameter.
      parameter_config: The dictionary containing information specific to the
        parameter in question. This is retrieved from request.parameters in
        the method config.
  
    Raises:
      EnumRejectionError: If the given value is not among the accepted
        enum values in the field parameter.