Good point. Unless anyone else disagrees, I think I'm going to make this `S <: WritableStore`. The reason being the placement of encoding of semantics. Since the encoding of the disassociation of a value from its key is provided in terms of the Option signature in WritableStores, I think it also makes sense encode the ephemeral associations between values and keys in terms of a WritableStore. I could go either way though. Let me know if you have a strong opinion on moving this back to Store and I'll do so.