<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def with_subtype(self, subtype):
 """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 """

        self._validate_subtype(subtype)
        self.subtype = subtype
        return self