Before revision:  This raises the question of how they arise in real-world systems like digital computers. We show that any deterministic map over some "visible" states can be implemented with a master equation--but only if additional "hidden" states are dynamically coupled to those visible states .
After revision:  This raises the question of how they can occur in the real world. Here we show how any function over some "visible" states can be implemented with a master equation--but only if additional "hidden" states are dynamically coupled to those visible states .