Packet.UpdateDNS update packets.
equal_prereq a b is true if a and b are equal, false otherwise.
equal_update a b is true if a is equal to b, false otherwise.
type t = prereq list Domain_name.Map.t * update list Domain_name.Map.tThe type of a DNS update: a map indexed by domain name with a list of prerequisites, and a map indexed by domain name of a list of updates.
val empty : tempty is the empty update.