type t = Soa.t * [ `Empty | `Full of Name_rr_map.t | `Difference of Soa.t * Name_rr_map.t * Name_rr_map.t ]
The type of an incremental zone transfer between two serials, consisting of the new start of authority and:
- Empty, if there are no changes
- Full zone transfer, same as AXFR
- Difference, with the old start of authority, the resource record sets to be removed, and the resource record sets to be added.