code: def has_variant_regions(items, base_file, chrom=None):
      
      if chrom:
          all_vrs = _get_variant_regions(items)
          if len(all_vrs) > 0:
              test = shared.subset_variant_regions(tz.first(all_vrs), chrom, base_file, items)
              if test == chrom:
                  return False
      return True
docs: Determine if we should process this chromosome: needs variant regions defined.