e.g. "coe [_ ⇒ A] @p @q s" should immediately reduce to "s", but if the "_ ⇒ A" happened to use an SY it didn't. this will still happen if a wrong SY sneaks in but the alternative is re-traversing the term over and over every time whnf runs
injecting from m to (n+m) is just id ::: id ::: ... ::: shift n. specifically, injecting from 0 is just the shift. so.
