This commit doesn't change any functionality, but instead changes the
representation of `CoveringNode` to make the implementation simpler (as
well as planned future additions). By putting the found node last in the
list of ancestors (now just generically called `nodes`), we reduce the
amount of special case handling we need.
The downside is that the representation now allows invalid states (a
`CoveringNode` with no elements). But I think this is well mitigated by
encapsulation.
65f32edbc7