This would have been useful for one of Jane Street's recent puzzles: https://www.janestreet.com/puzzles/tit-for-tat-index/ https://www.janestreet.com/puzzles/ti

This would have been useful for one of Jane Street's recent puzzles: https://www.janestreet.com/puzzles/tit-for-tat-index/ https://www.janestreet.com/puzzles/tit-for-tat-index/ I ended up just grepping through a large dictionary instead.