code: def _short_chrom(self, chrom):
          default_allowed = set(["X"])
          allowed_chroms = set(getattr(config, "goleft_indexcov_config", {}).get("chromosomes", []))
          chrom_clean = chrom.replace("chr", "")
          try:
              chrom_clean = int(chrom_clean)
          except ValueError:
              if chrom_clean not in default_allowed and chrom_clean not in allowed_chroms:
                  chrom_clean = None
          if allowed_chroms:
              if chrom in allowed_chroms or chrom_clean in allowed_chroms:
                  return chrom_clean
          elif isinstance(chrom_clean, int) or chrom_clean in default_allowed:
              return chrom_clean