jsv.property('is inverse to encodePathSegments', jsv.array(jsv.string), segments =>
   segments.length === 1 && segments[0] === '' ? true
     : deepEq(decodePathSegments(encodePathSegments(segments)), segments)

export function saveSessionOnDb(
  state: ServerSessionState,
  now: number,
  msess: Session | null,
  dec: DecomposedSession
): StorageTx<Session | null> {
  const storage = state.storage;
  const ctx = storage.tx;
  if (msess == null && dec.authId == null && isEmpty(dec.decomposed)) return ctx.of(null);

  if (msess != null && state.timeoutResolution !== null && msess.authId === dec.authId && deepEq(msess.data, dec.decomposed)
      && Math.abs(msess.accessedAt - now) < (state.timeoutResolution * 1000)) {
    return ctx.of(msess);
  }

  if (msess == null) {
    return ctx.liftTask(generateSessionId)
      .chain(id => {
        const sess = createSession(id, dec.authId, dec.decomposed, now, now);
        return storage.insert(sess).map(() => sess);
      });
  }
  const sess = createSession(msess.id, dec.authId, dec.decomposed, msess.createdAt, now);
  return storage.replace(sess).map(() => sess);
}

function tEquals<A>(xs: [A, A]) {
  return deepEq(xs[0], xs[1]);
}

 jsv.property('filter keep all', intMapArb(jsv.number), im =>
   P.deepEq(im, IM.filter(() => true, im))