docs: def clear(self, context=None):
          """ Delete all data from the graph. """
          context = URIRef(context).n3() if context is not None else '?g'
          query = """
              DELETE { GRAPH %s { ?s ?p ?o } } WHERE { GRAPH %s { ?s ?p ?o } }
          """ % (context, context)
          self.parent.graph.update(query)