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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 previousExon(self) : 		"""Returns the previous exon of the transcript, or None if there is none"""

		
		if self.number == 0 :
			return None
		
		try :
			return self.transcript.exons[self.number-1]
		except IndexError :
			return None