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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 is_valid(self, context, sid):
 		"""Identify if the given session ID is currently valid. 		 		Return True if valid, False if explicitly invalid, None if unknown. 		"""

		
		record = self._Document.find_one(sid, project=('expires', ))
		
		if not record:
			return
		
		return not record._expired