def connected_to(self, vertex_id):
        """
        Parameters
        -----------
        vertex_id : int
          Get what `vertex_id` is connected to.

        Returns
        -----------
        int|None, the vertex id connected to the
          input `vertex_id` in this edge, as long as `vertex_id` is
          one of the vertices connected by this edge.
        """
        if vertex_id not in self.edge:
            return None
        connected_to = (self.edge[1] if self.edge[0] == vertex_id
                        else self.edge[0])
        return connected_to