function opIdDelta(id1, id2, delta = 1) {
  const parsed1 = parseOpId(id1), parsed2 = parseOpId(id2)
  return parsed1.actorId === parsed2.actorId && parsed1.counter + delta === parsed2.counter
}
Returns true if the two given operation IDs have the same actor ID, and the counter of `id2` is
exactly `delta` greater than the counter of `id1`.