Resolved `FIXME`s: Reworked functions with in-band error reporting to distinguish their errors from valid empty sets. Also added a few assertions.
Resolved `FIXME`s: Reworked functions with in-band error reporting to distinguish their errors from valid empty sets. Also added a few assertions.

katef/libfsm,katef/libfsm,katef/libfsm,katef/libfsm,katef/libfsm