When we get our unit test infrastructure in better shape, it seems we
should have a test like this:
crudeSift xs == fancySift xs if xs only contains crude siftable things
fancySift doesn't exist right now (except maybe as helpers to
siftForPending). It's also not clear to me yet if fancySift can be
defined as something that doesn't use crudeSift (it seems like it
should)