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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_vts_results_dir(self, vts):
 """ Given a `VergetTargetSet`, prepare its results dir. """

    vt_set_results_dir = os.path.join(self.versioned_workdir, 'results', vts.cache_key.hash)
    safe_mkdir(vt_set_results_dir)
    return vt_set_results_dir