docs: 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