Notice how this changes if we pick a different vertex for the root. If \(a\) is the root, then its lone child is \(c\text{,}\) which also has only one child, namely \(e\text{.}\) We would then have \(f\) the child of \(e\) (instead of the other way around), and \(f\) is the descendant of \(a\text{,}\) instead of the ancestor. \(f\) and \(g\) are now siblings.
in-context