func (x *intervalNode) successor() *intervalNode {
	if x.right != nil {
		return x.right.min()
	}
	y := x.parent
	for y != nil && x == y.right {
		x = y
		y = y.parent
	}
	return y
}
func (x *intervalNode) successor() *intervalNode {
	if x.right != nil {
		return x.right.min()
	}
	y := x.parent
	for y != nil && x == y.right {
		x = y
		y = y.parent
	}
	return y
}
// successor is the next in-order node in the tree