code: def infer_missing_backwards_edge(graph, u, v, k):
      
      if u in graph[v]:
          for attr_dict in graph[v][u].values():
              if attr_dict == graph[u][v][k]:
                  return
  
      graph.add_edge(v, u, key=k, **graph[u][v][k])
docs: Add the same edge, but in the opposite direction if not already present.
  
      :type graph: pybel.BELGraph
      :type u: tuple
      :type v: tuple
      :type k: int