code: def xrefs(self, nid, bidirectional=False):
          
          if self.xref_graph is not None:
              xg = self.xref_graph
              if nid not in xg:
                  return []
              if bidirectional:
                  return list(xg.neighbors(nid))
              else:
                  return [x for x in xg.neighbors(nid) if xg[nid][x][0]['source'] == nid]
  
          return []
docs: Fetches xrefs for a node
  
          Arguments
          ---------
          nid : str
              Node identifier for entity to be queried
          bidirection : bool
              If True, include nodes xreffed to nid
  
          Return
          ------
          list[str]