code: def parallel_variantcall_region(samples, run_parallel):
      
      to_process, extras = _dup_samples_by_variantcaller(samples)
      split_fn = _split_by_ready_regions(".vcf.gz", "work_bam", get_variantcaller)
      samples = _collapse_by_bam_variantcaller(
          grouped_parallel_split_combine(to_process, split_fn,
                                         multi.group_batches, run_parallel,
                                         "variantcall_sample", "concat_variant_files",
                                         "vrn_file", ["region", "sam_ref", "config"]))
      return extras + samples
docs: Perform variant calling and post-analysis on samples by region.