Why does Parser.Pike accept a bunch of backquote identifiers that don't correspond to any lfuns and that pike itself doesn't accept?
The reason I found this is because post_modules/_ADT/sequence.cmod tries to define an lfun `-=. That's currently meaningless since it will never be called, but perhaps the intention is to prepare for a future extension.
Anyway, it means that 7.5 currently can't use a 7.4 Pike for precompilation, which I think should be possible. Maybe the stuff in precompile.sh.in that made older pikes use the latest parser modules should be resurrected.