code: def infer_missing_two_way_edges(graph):
      
      for u, v, k, d in graph.edges(data=True, keys=True):
          if d[RELATION] in TWO_WAY_RELATIONS:
              infer_missing_backwards_edge(graph, u, v, k)
docs: Add edges to the graph when a two way edge exists, and the opposite direction doesn't exist.
  
      Use: two way edges from BEL definition and/or axiomatic inverses of membership relations
  
      :param pybel.BELGraph graph: A BEL graph