def remove_vertex (v)
      @vertex_dict.delete(v)
      @predecessor_dict.delete(v)
      # remove v from all adjacency lists

      @vertex_dict.each_value { |adjList| adjList.delete(v) }
      @predecessor_dict.each_value { |predlist| predlist.delete(v) }
    end
def remove_vertex (v)
      @vertex_dict.delete(v)
      @predecessor_dict.delete(v)
      # remove v from all adjacency lists

      @vertex_dict.each_value { |adjList| adjList.delete(v) }
      @predecessor_dict.each_value { |predlist| predlist.delete(v) }
    end
See MutableGraph#remove_vertex.