Zipper With Empty Context Does Not Define flatMap

[djspiewak]

When a Zipper is created with an empty context (e.g. via Group#toZipper), the resulting Zipper does not define a working flatMap. If this were just a result of toZipper, then it would be excusable. Unfortunately, this arises when actually using Zipper normally:

val g: Group[Node] = ...
(g \ "foo").unselect flatMap { e => Some(e) }         // => Group(), regardless of what g is

This appears to be a product of the fact that flatMap builds its result by mapping over the zipper context. Since that context is empty after unselect (in this case), the result will always be Group().