I've skimmed over section 3.3 of [Altidor et al (PLDI 2011)](http://people.cs.umass.edu/~jaltidor/variance_pldi11.pdf).  I do not immediately see a simple example of a region parameter that will be bivariant *and* useful, but I haven't thought that hard about it yet.

I don't mind keeping the enum the way it is, but I'd appreciate it if you either explicitly noted whether `Bivariant` is here just to keep a nice variance lattice, or if there is a concrete case for *region* variance that having it addresses.