The AIJ translator paper associates layers with axiom *rules*, not with axiom
*variables*. This is more general because it allows for non-binary derived
variables. (In our implementation's version, non-binary derived variables would
either be trivial, i.e. certain values could never be achieved, or they'd
violate the layering property in the AIJ translator paper.)
For people who directly generate FDR tasks rather than go through our
translator, we should either support the more general case, or at least properly
document the restriction of derived variables to the binary case.