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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 index(in_bam, config, check_timestamp=True):
 """Index a BAM file, skipping if index present. Centralizes BAM indexing providing ability to switch indexing approaches. """

    assert is_bam(in_bam), "%s in not a BAM file" % in_bam
    index_file = "%s.bai" % in_bam
    alt_index_file = "%s.bai" % os.path.splitext(in_bam)[0]
    if check_timestamp:
        bai_exists = utils.file_uptodate(index_file, in_bam) or utils.file_uptodate(alt_index_file, in_bam)
    else:
        bai_exists = utils.file_exists(index_file) or utils.file_exists(alt_index_file)
    if not bai_exists:
        # Remove old index files and re-run to prevent linking into tx directory
        for fname in [index_file, alt_index_file]:
            utils.remove_safe(fname)
        samtools = config_utils.get_program("samtools", config)
        num_cores = config["algorithm"].get("num_cores", 1)
        with file_transaction(config, index_file) as tx_index_file:
            cmd = "{samtools} index -@ {num_cores} {in_bam} {tx_index_file}"
            do.run(cmd.format(**locals()), "Index BAM file: %s" % os.path.basename(in_bam))
    return index_file if utils.file_exists(index_file) else alt_index_file