what has been fetched from the sidecar SESSION `epoch`, by driver index. A wave is here when it has been asked for, whatever window it was asked over; whether that window is the one being drawn is the caller's question and is answered by `hasData`. **The epoch is what makes invariant D4 true rather than intended.** A driver index names a different signal in the next build, and a fetch already in the air when one starts still lands - so without a session on the cache itself, the previous build's samples were written under indices the new build had reused and drawn under the new signal's name. The comment here used to point at a check in the update function; there was no such check, and there is nowhere for one to be, because this is written from inside a promise where the model is not reachable. Saying which session the cache is OF settles it where the writing happens.