[xigoi]

> By direction, I mean that if you define "a grandparent is the parent of a parent", you can now use that rule not just to evaluate whether one person is a parent (or to find all grandparents) but also to conclude that if you know someone is a grandparent, and they are the parent of some one, then that person is someone's parent.

Not necessarily.

[f1shy]

Could you please expand? Because I have the same state of knowledge of the previous poster. I would like to learn if I'm wrong.

[xigoi]

Someone who is a grandparent may also have other children who do not have children.