def to_dot(buffer='')
      buffer << "digraph G {\n"
      buffer << "  graph[#{to_dot_attributes(self.to_h(true))}]\n"
      each_vertex do |v|
        buffer << "  V#{v.index} [#{to_dot_attributes(v.to_h(true))}]\n"
      end
      each_edge do |e|
        buffer << "  V#{e.source.index} -> V#{e.target.index} [#{to_dot_attributes(e.to_h(true))}]\n"
      end
      buffer << "}\n"
    end
def to_dot(buffer='')
      buffer << "digraph G {\n"
      buffer << "  graph[#{to_dot_attributes(self.to_h(true))}]\n"
      each_vertex do |v|
        buffer << "  V#{v.index} [#{to_dot_attributes(v.to_h(true))}]\n"
      end
      each_edge do |e|
        buffer << "  V#{e.source.index} -> V#{e.target.index} [#{to_dot_attributes(e.to_h(true))}]\n"
      end
      buffer << "}\n"
    end
Standard exports 
 Encodes this graph for dot graphviz