// For i -> j -> k, if i -> k exists, delete it.
	for _, i := range g.Nodes {
		for _, j := range g.Nodes {
			if g.HasEdge(i, j) {
				for _, k := range g.Nodes {
					if g.HasEdge(j, k) {
						g.DelEdge(i, k)
					}
				}
			}
		}
	}

	g.Nodes = append(g.Nodes, label)
	g.edges[label] = map[string]bool{}
	return true
}

func (g *Graph) AddEdge(from, to string) {
	g.edges[from][to] = true
}

func (g *Graph) DelEdge(from, to string) {
	delete(g.edges[from], to)
}

func (g *Graph) HasEdge(from, to string) bool {
	return g.edges[from] != nil && g.edges[from][to]
}

func (g *Graph) Edges(from string) []string {