If return is given an empty sequence, pigpen fails later with a bizarre,
seemingly unrelated arity exception because it can't figure out the
number of expected fields in the input. I'm not sure what a real
solution is, so at least fail early with a clear assertion.
If return is given an empty sequence, pigpen fails later with a bizarre, seemingly unrelated arity exception because it can't figure out the number of expected fields in the input. I'm not sure what a real solution is, so at least fail early with a clear assertion.