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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def search_variants( self, variant_set_id, start=None, end=None, reference_name=None, call_set_ids=None):
 """ Returns an iterator over the Variants fulfilling the specified conditions from the specified VariantSet. :param str variant_set_id: The ID of the :class:`ga4gh.protocol.VariantSet` of interest. :param int start: Required. The beginning of the window (0-based, inclusive) for which overlapping variants should be returned. Genomic positions are non-negative integers less than reference length. Requests spanning the join of circular genomes are represented as two requests one on each side of the join (position 0). :param int end: Required. The end of the window (0-based, exclusive) for which overlapping variants should be returned. :param str reference_name: The name of the :class:`ga4gh.protocol.Reference` we wish to return variants from. :param list call_set_ids: Only return variant calls which belong to call sets with these IDs. If an empty array, returns variants without any call objects. If null, returns all variant calls. :return: An iterator over the :class:`ga4gh.protocol.Variant` objects defined by the query parameters. :rtype: iter """

        request = protocol.SearchVariantsRequest()
        request.reference_name = pb.string(reference_name)
        request.start = pb.int(start)
        request.end = pb.int(end)
        request.variant_set_id = variant_set_id
        request.call_set_ids.extend(pb.string(call_set_ids))
        request.page_size = pb.int(self._page_size)
        return self._run_search_request(
            request, "variants", protocol.SearchVariantsResponse)