code: def get_related_node(self, node, relation):
          
          G = self.G
          for edge in G.edges(node):
              to = edge[1]
  
              to_relation = G.edges[node, to]['relation']
              if to_relation == relation:
                  return to
          return None
docs: Looks for an edge from node to some other node, such that the edge
          is annotated with the given relation. If there exists such an edge,
          returns the name of the node it points to. Otherwise, returns None.