// Delete old edges from source to target.
	for i, e := range source.Edges {

	found := false
	// Look for the old edge and update it if it is found.
	for _, e := range source.Edges {

			source.Edges = append(source.Edges[:i], source.Edges[i+1:]...)

			found = true
			e.Change = change
			e.Flag = ne.Flag

	for i, e := range target.ReverseEdges {
		if e.Change == introducedBy && e.From == source && e.To == target {
			target.ReverseEdges = append(target.ReverseEdges[:i], target.ReverseEdges[i+1:]...)
			break
		}

	if !found {
		makeEdge(change, ne.Flag, source, target)

	makeEdge(change, ne.Flag, source, target)