code: final void unipush(Completion c) {
      if (c != null) {
        while (!tryPushStack(c)) {
          if (result != null) {
            lazySetNext(c, null);
            break;
          }
        }
        if (result != null)
          c.tryFire(SYNC);
      }
    }
docs: Pushes the given completion unless it completes while trying. Caller should first check that
  result is null.