code: def _check_aligner(item):
      
      allowed = set(list(alignment.TOOLS.keys()) + [None, False])
      if item["algorithm"].get("aligner") not in allowed:
          raise ValueError("Unexpected algorithm 'aligner' parameter: %s\n"
                           "Supported options: %s\n" %
                           (item["algorithm"].get("aligner"), sorted(list(allowed))))
docs: Ensure specified aligner is valid choice.