* concurrent with this method, the returned value is not guaranteed to be the final
   * value occurring before the reset.
   *
   * @return the sum
   */
  public long sumThenReset() {
    long sum = base;
    Cell[] as = cells;
    base = 0L;
    if (as != null) {
      int n = as.length;
      for (int i = 0; i < n; ++i) {
        Cell a = as[i];
        if (a != null) {