docs: 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