func TestTransitiveReduction(t *testing.T) {
	t.Run("diamond", func(t *testing.T) {
		g := mustParse(t, diamond)
		g.TransitiveReduction()
		wantEdges(t, g, "b->a c->a d->b d->c")
	})
	t.Run("chain", func(t *testing.T) {
		const chain = `NONE < a < b < c < d; a, d < e;`
		g := mustParse(t, chain)
		g.TransitiveReduction()
		wantEdges(t, g, "e->d d->c c->b b->a")
	})

		visit(root)
	}
	for i, j := 0, len(topo)-1; i < j; i, j = i+1, j-1 {
		topo[i], topo[j] = topo[j], topo[i]
	}
	return topo
}

// TransitiveReduction removes edges from g that are transitively
// reachable. g must be transitively closed.
func (g *Graph) TransitiveReduction() {
	// 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) {

		os.Exit(2)
	}

	g, err := dag.Parse(ranks)
	if err != nil {
		log.Fatal(err)
	}

	var out []byte
	if *flagDot {
		var b bytes.Buffer
		g.TransitiveReduction()
		// Add cyclic edges for visualization.
		for k := range cyclicRanks {
			g.AddEdge(k, k)
		}
		// Reverse the graph. It's much easier to read this as
		// a "<" partial order than a ">" partial order. This