code: def tiered_alignment(in_bam, tier_num, multi_mappers, extra_args,
                       genome_build, pair_stats,
                       work_dir, dirs, config):
      
      nomap_fq1, nomap_fq2 = select_unaligned_read_pairs(in_bam, "tier{}".format(tier_num),
                                                         work_dir, config)
      if nomap_fq1 is not None:
          base_name = "{}-tier{}out".format(os.path.splitext(os.path.basename(in_bam))[0],
                                            tier_num)
          config = copy.deepcopy(config)
          dirs = copy.deepcopy(dirs)
          config["algorithm"]["bam_sort"] = "queryname"
          config["algorithm"]["multiple_mappers"] = multi_mappers
          config["algorithm"]["extra_align_args"] = ["-i", int(pair_stats["mean"]),
                                                 int(pair_stats["std"])] + extra_args
          out_bam, ref_file = align_to_sort_bam(nomap_fq1, nomap_fq2,
                                                lane.rg_names(base_name, base_name, config),
                                                genome_build, "novoalign",
                                                dirs, config,
                                                dir_ext=os.path.join("hydra", os.path.split(nomap_fq1)[0]))
          return out_bam
      else:
          return None
docs: Perform the alignment of non-mapped reads from previous tier.