One point I think he should have emphasized more is that concatenation with the empty language gives you the empty language.Then it becomes obvious why no arrow is the same as the arrow with and the empty language.####[['NULL', 'Instructor', 'NEG'], ['concatenation', 'Mathematical_Related_Concept', 'NEU'], ['empty language', 'Mathematical_Related_Concept', 'NEU'], ['arrow', 'Mathematical_Related_Concept', 'NEU']]