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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 _get_rev_num(self, rev=None):
 		""" 		Determine the revision number for a given revision specifier. 		"""

		# first, determine the numeric ID
		cmd = ['identify', '--num']
		# workaround for #4
		cmd.extend(['--config', 'defaults.identify='])
		if rev:
			cmd.extend(['--rev', rev])
		res = self._invoke(*cmd)
		return res.strip()