code: def _filter_by_background(base_name, back_samples, gt_vcfs, data):
      
      filtname = "InBackground"
      filtdoc = "Variant also present in background samples with same genotype"
      orig_vcf = gt_vcfs[base_name]
      out_file = "%s-backfilter.vcf" % (utils.splitext_plus(orig_vcf)[0])
      if not utils.file_exists(out_file) and not utils.file_exists(out_file + ".gz"):
          with file_transaction(data, out_file) as tx_out_file:
              with utils.open_gzipsafe(orig_vcf) as in_handle:
                  inp = vcf.Reader(in_handle, orig_vcf)
                  inp.filters[filtname] = vcf.parser._Filter(filtname, filtdoc)
                  with open(tx_out_file, "w") as out_handle:
                      outp = vcf.Writer(out_handle, inp)
                      for rec in inp:
                          if _genotype_in_background(rec, base_name, back_samples):
                              rec.add_filter(filtname)
                          outp.write_record(rec)
      if utils.file_exists(out_file + ".gz"):
          out_file = out_file + ".gz"
      gt_vcfs[base_name] = vcfutils.bgzip_and_index(out_file, data["config"])
      return gt_vcfs
docs: Filter base samples, marking any also present in the background.