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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 rdf_source(self, aformat="turtle"):
 		""" 		Serialize graph using the format required 		"""

		if aformat and aformat not in self.SUPPORTED_FORMATS:
			return "Sorry. Allowed formats are %s" % str(self.SUPPORTED_FORMATS)
		if aformat == "dot":
			return self.__serializedDot()
		else:
			# use stardard rdf serializations
			return self.rdflib_graph.serialize(format=aformat)