Uses rdflib.compare diff, https://github.com/RDFLib/rdflib/blob/master/rdflib/compare.py
		When a resource is retrieved, the graph retrieved and parsed at that time is saved to self.rdf._orig_graph,
		and all local modifications are made to self.rdf.graph.  This method compares the two graphs and returns the diff
		in the format of three graphs:

			overlap - triples SHARED by both
			removed - triples that exist ONLY in the original graph, self.rdf._orig_graph
			added - triples that exist ONLY in the modified graph, self.rdf.graph

		These are used for building a sparql update query for self.update.

		Args:
			None

		Returns:
			None: sets self.rdf.diffs and adds the three graphs mentioned, 'overlap', 'removed', and 'added'

def _diff_graph(self):

		'''
		Uses rdflib.compare diff, https://github.com/RDFLib/rdflib/blob/master/rdflib/compare.py
		When a resource is retrieved, the graph retrieved and parsed at that time is saved to self.rdf._orig_graph,
		and all local modifications are made to self.rdf.graph.  This method compares the two graphs and returns the diff
		in the format of three graphs:

			overlap - triples SHARED by both
			removed - triples that exist ONLY in the original graph, self.rdf._orig_graph
			added - triples that exist ONLY in the modified graph, self.rdf.graph

		These are used for building a sparql update query for self.update.

		Args:
			None

		Returns:
			None: sets self.rdf.diffs and adds the three graphs mentioned, 'overlap', 'removed', and 'added'
		'''

		overlap, removed, added = graph_diff(
			to_isomorphic(self.rdf._orig_graph),
			to_isomorphic(self.rdf.graph))
		diffs = SimpleNamespace()
		diffs.overlap = overlap
		diffs.removed = removed
		diffs.added = added
		self.rdf.diffs = diffs