code: def merge(self, ontologies):
          
          if self.xref_graph is None:
              self.xref_graph = nx.MultiGraph()
          logger.info("Merging source: {} xrefs: {}".format(self, len(self.xref_graph.edges())))
          for ont in ontologies:
              logger.info("Merging {} into {}".format(ont, self))
              g = self.get_graph()
              srcg = ont.get_graph()
              for n in srcg.nodes():
                  g.add_node(n, **srcg.node[n])
              for (o,s,m) in srcg.edges(data=True):
                  g.add_edge(o,s,**m)
              if ont.xref_graph is not None:
                  for (o,s,m) in ont.xref_graph.edges(data=True):
                      self.xref_graph.add_edge(o,s,**m)
docs: Merges specified ontology into current ontology