UML22 — Section: 15.3.12

The problem is with the definition of the ancestor query on page 559. I believe that the algorithm, as stated, determines whether s1 is an ancestor of s2, not whether s2 is an ancestor of s1.

This method is testing that s2 is nested within s1, and such s1 would be the ancestor. The method description is therefore wrong and needs to be fixed as suggested.
Furthermore, the check that s1 has a container has no bearing on whether s2 is contained within it. And the parameters of the function accept states but we are passing the container which will be a region.