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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 samtoolsMpileup(outFile, referenceFile, alignmentFile, executor):
 """ Use samtools mpileup to generate VCF. @param outFile: The C{str} name to write the output to. @param referenceFile: The C{str} name of the FASTA file with the reference sequence. @param alignmentFile: The C{str} name of the SAM or BAM alignment file. @param executor: An C{Executor} instance. """

    executor.execute(
        'samtools mpileup -u -v -f %s %s > %s' %
        (referenceFile, alignmentFile, outFile))