code: def _joint_calling(items):
      
      jointcaller = tz.get_in(("config", "algorithm", "jointcaller"), items[0])
      if jointcaller:
          assert len(items) == 1, "Can only do joint calling preparation with GATK with single samples"
          assert tz.get_in(("metadata", "batch"), items[0]) is not None, \
              "Joint calling requires batched samples, %s has no metadata batch." % dd.get_sample_name(items[0])
      return jointcaller
docs: Determine if this call feeds downstream into joint calls.