func (ivt *IntervalTree) Visit(ivl Interval, ivv IntervalVisitor) {
	ivt.root.visit(&ivl, func(n *intervalNode) bool { return ivv(&n.iv) })
}
func (ivt *IntervalTree) Visit(ivl Interval, ivv IntervalVisitor) {
	ivt.root.visit(&ivl, func(n *intervalNode) bool { return ivv(&n.iv) })
}
// Visit calls a visitor function on every tree node intersecting the given interval.
// It will visit each interval [x, y) in ascending order sorted on x.