func (d *ltDom) eval(v vName) vName {
	if d.ancestor[v] == -1 {
		return v
	}
	d.compress(v)
	return d.labels[v]
}
func (d *ltDom) eval(v vName) vName {
	if d.ancestor[v] == -1 {
		return v
	}
	d.compress(v)
	return d.labels[v]
}
// eval is EVAL from the papers.