def remove_edge (u, v)
      @vertex_dict[u].delete(v) unless @vertex_dict[u].nil?
      @predecessor_dict[v].delete(u) unless @predecessor_dict[v].nil?
    end
def remove_edge (u, v)
      @vertex_dict[u].delete(v) unless @vertex_dict[u].nil?
      @predecessor_dict[v].delete(u) unless @predecessor_dict[v].nil?
    end
See MutableGraph::remove_edge.