checkState(state != State.FAILED);
    switch (state) {
      case DONE:
        return false;
      case READY:
        return true;
      default:
    }
    return tryToComputeNext();
  }

  private boolean tryToComputeNext() {
    state = State.FAILED; // temporary pessimism
    next = computeNext();
    if (state != State.DONE) {
      state = State.READY;
      return true;
    }