Sorry @Noah K,blitzed the question for now since I may have made a mistake, but the pattern at play here is that the upstream solid may read a number of input files of a certain type and transform them into outputs for the downstream solid. We're overriding the intermediate storage so the outputs can be tested by our system tester, so we don't want to just pickle a collection - we want a variable number of outputs and hence we'll get a variable number of inputs downstream.