Aha. That explains it. So 'back' is used as an identifier even if you're not using gamebook mode?
I seem to have a knack of finding these things unintentionally. Two jams ago, it was 'root', remember? As you have this silly system where the id determines the adjective and noun, I had to find a workaround for 'root'. The workaround was to append a number (e.g. 'root_01'), but this has its own issues (reported in the last jam) or to prepend a junk adjective (e.g. 'dummy_root').
Anyway, I now know what 'back' is.