Netflix/PigPen — pull request #34 by daveray (2014-06-15 22:46:27+00:00)
Title: Assert that data passed to return is not empty

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.