rw [← get?_eq_get]
case mk
n j : Nat
hj : j < n
⊢ some (get (fins n) { val := j, isLt := ⋯ }) = some { val := j, isLt := ⋯ }
case mk
n j : Nat
hj : j < n
⊢ get? (fins n) j = some { val := j, isLt := ⋯ }