Interesting they used the logical equivalence operator. I suspect they mean "Midi Files ⊂ Useless".(That's the is-a-subset-of character.)Otherwise, they would be saying that all useless things are also midi files, which I don't think is correct.
How very observant of you Bill, and so very true! The first thing I thought of when I read your comment was this "Midi Files < Useless" which sounds quote derogatory when spoken out loud :)