private @Nullable Entry<E> prevEntry;

      @Override
      public boolean hasNext() {
        if (current == null) {
          return false;
        } else if (range.tooHigh(current.getElement())) {
          current = null;
          return false;
        } else {
          return true;
        }
      }

      @Override
      public Entry<E> next() {

    // durations longer than approximately +/- 292 years).
    try {
      return duration.toNanos();
    } catch (ArithmeticException tooBig) {
      return duration.isNegative() ? Long.MIN_VALUE : Long.MAX_VALUE;
    }
  }

    // durations longer than approximately +/- 292 years).
    try {
      return duration.toNanos();
    } catch (ArithmeticException tooBig) {
      return duration.isNegative() ? Long.MIN_VALUE : Long.MAX_VALUE;
    }
  }