Revocation/Retraction API

[arrdem]

Datomic and PLDB both have the concept of "retracting" a fact which has been inserted into the store. This allows for deletion even from "value" storage with the understanding that it's an unusual operation.

Shelving at present has no delete, not even on records.
It's worth thinking about whether this makes any sort of sense to implement, given Shelving's current focus on modeling spec'd data.