docs: def _get_generator(parser, extract, keep, check_maf):
      """Generates the data (with extract markers and keep, if required."""
      if extract is not None:
          parser = Extractor(parser, names=extract)
  
      for data in parser.iter_genotypes():
          data.genotypes = data.genotypes[keep]
  
          # Checking the MAF, if required
          if check_maf:
              data.code_minor()
  
          yield data