const outcome = this._step();
if (outcome === 'quiesce') { return; }
// 'progress': read the next event so the embedding can pick a
// per-event primitive. If there is none, loop and let the next
// `_step` decide between 'park' and 'quiesce'.
const next = this._clock.peekNext();
if (next === undefined) { continue; }
const choice = this._embedding(next, this._drive);