WCOJ - Datalog and GenericJoin

Edit(07-06-2026): I have realized that the OrPrefixExtender below is not quite right, as variable instantiations from other or branches can “leak” into extensions in lower variable levels. Or branches need to get executed in isolation, at least for the variables they participate in. The other option is to check the branch again when it gets extended for a particular prefix, but that seems…