Add NOTE comments.
Add missing static specifiers.
Enable subexpression returning Vector<Text>.
Add a test for subexpression returning Vector<Text>.
Add Expression::is_row_id() and Expression::is_score(). (#119)
Add tests for Expression::is_row_id/score(). (#119)
Specialize Sorter for Score. (#119)
Add a test for sortings records in score order. (#119)
Remove wrong comments.