code: def readsGenerator(self, request):
          
          if not request.reference_id:
              raise exceptions.UnmappedReadsNotSupported()
          if len(request.read_group_ids) < 1:
              raise exceptions.BadRequestException(
                  "At least one readGroupId must be specified")
          elif len(request.read_group_ids) == 1:
              return self._readsGeneratorSingle(request)
          else:
              return self._readsGeneratorMultiple(request)
docs: Returns a generator over the (read, nextPageToken) pairs defined
          by the specified request