code: def _subset_regions(region, base_file, items):
      
      variant_regions = bedutils.population_variant_regions(items, merged=True)
      target = pshared.subset_variant_regions(variant_regions, region, base_file, items)
      if isinstance(target, six.string_types) and os.path.isfile(target):
          return target
      else:
          return bamprep.region_to_gatk(target)
docs: Subset to a BED file (or genomic region) for calling.