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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _short_chrom(self, chrom):
 """Plot standard chromosomes + X, sorted numerically. Allows specification from a list of chromosomes via config for non-standard genomes. """

        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