* so its return value will get used naturally.
     */
    @SuppressWarnings("CanIgnoreReturnValueSuggester")
    ImmutableTableCollectorState<R, C, V> combine(
        ImmutableTableCollectorState<R, C, V> other, BinaryOperator<V> merger) {
      for (MutableCell<R, C, V> cell : other.insertionOrder) {
        put(cell.getRowKey(), cell.getColumnKey(), cell.getValue(), merger);