assert(none_of(row_items.begin(), row_items.end(),
[](const shared_ptr<RowItem> &r) { return !r; }));
stable_sort(row_items.begin(), row_items.end(),
[](const shared_ptr<RowItem> &a, const shared_ptr<RowItem> &b) {
assert(none_of(row_items.begin(), row_items.end(),
[](const shared_ptr<RowItem> &r) { return !r; }));
stable_sort(row_items.begin(), row_items.end(),
[](const shared_ptr<RowItem> &a, const shared_ptr<RowItem> &b) {