test('writing unblocks reads', function* () {
      const chan: Q.Chan<number> = yield Q.newChan;
      yield T.forkTask(T.apSecond(T.delay(20), Q.writeChan(chan, 1)));
      const r: P.Either<void, number> = yield compete(T.delay(50), Q.readChan(chan));
      assert.ok(P.isRight(r));

      return T.pure(void 0);
    });

 return P.list.foldr((a, b) => T.apSecond(fn(a).parallel(), b), T.Parallel.of(void 0), xs).sequential();

 function go(x: number): T.Task<void> {
   return x <= 0 ? T.pure(void 0) : T.apSecond(task, go(x - 1));
 }

 .chain(oldHole =>
   T.apSecond(
     AV.putAVar(oldHole, new ChannelItem(val, newHole)),
     AV.putAVar(chan.write, newHole)
   )