  The data for many useful bidirectional constructions in applied category
theory (optics, learners, games, quantum combs) can be expressed in terms of
diagrams containing "holes" or "incomplete parts", sometimes known as comb
diagrams. We give a possible formalization of what these circuits with
incomplete parts represent in terms of symmetric monoidal categories, using the
dinaturality equivalence relations arising from a coend. Our main idea is to
extend this formal description to allow for infinite circuits with holes
indexed by the natural numbers. We show how infinite combs over an arbitrary
symmetric monoidal category form again a symmetric monoidal category where
notions of delay and feedback can be considered. The constructions presented
here are still preliminary work.
