simp
case mk
n j : Nat
⊢ ∀ (hj : j < length (fins n)), get (fins n) { val := j, isLt := hj } = { val := ↑{ val := j, isLt := hj }, isLt := ⋯ }
case mk
n j : Nat
⊢ ∀ (hj : j < n), get (fins n) { val := j, isLt := ⋯ } = { val := j, isLt := ⋯ }