unfold fins.finsAux
case zero
n : Nat
hi : Nat.zero ≤ n
acc : List (Fin n)
⊢ fins.finsAux (Nat.succ n) Nat.zero ⋯ (map Fin.castSucc acc ++ [Fin.last n]) =
    map Fin.castSucc (fins.finsAux n Nat.zero hi acc) ++ [Fin.last n]
case zero
n : Nat
hi : Nat.zero ≤ n
acc : List (Fin n)
⊢ map Fin.castSucc acc ++ [Fin.last n] = map Fin.castSucc acc ++ [Fin.last n]