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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 _prepare_bam(bam_file, bed_file, config):
 """Remove regions from bed files"""

    if not bam_file or not bed_file:
        return bam_file
    out_file = utils.append_stem(bam_file, '_filter')
    bedtools = config_utils.get_program("bedtools", config)
    if not utils.file_exists(out_file):
        with file_transaction(out_file) as tx_out:
            cmd = "{bedtools} subtract -nonamecheck -A -a {bam_file} -b {bed_file} > {tx_out}"
            do.run(cmd.format(**locals()), "Remove blacklist regions from %s" % bam_file)
    return out_file