induction n
case mk
n j : Nat
hj : j < n
⊢ get? (fins n) j = some { val := j, isLt := ⋯ }
case mk.zero
j : Nat
hj : j < Nat.zero
⊢ get? (fins Nat.zero) j = some { val := j, isLt := ⋯ }

case mk.succ
j n✝ : Nat
n_ih✝ : ∀ (hj : j < n✝), get? (fins n✝) j = some { val := j, isLt := ⋯ }
hj : j < Nat.succ n✝
⊢ get? (fins (Nat.succ n✝)) j = some { val := j, isLt := ⋯ }