code: def with_subtype(self, subtype):
          
          self._validate_subtype(subtype)
          self.subtype = subtype
          return self
docs: Add a subtype segment
  
          Args:
              subtype (str): May be one of ``['a', 'c', 'r']``. See the
                  `Reference Definition <https://elections.democracyclub.org.uk/reference_definition>`_.
                  for valid election type/subtype combinations.
  
          Returns:
              IdBuilder
  
          Raises:
              ValueError