Will layers store whiteouts (a list of files to remove) in addition to added files?

If I remove /foo in a container, and export those changes into a new layer, what happens when I run a new container using that layer? Either a) /foo will be present (whiteouts are not stored in the layer), or b) /foo will not be present (whiteouts are stored in the layer).

Question 1: are whiteouts needed? Are certain use cases only possible with whiteouts?

Question 2: if we do need whiteouts in layer, what format should we use? We could use AUFS's convention - effectively making aufs part of the api, instead of just an implementation detail. Do we want that?