simpa using this
n : Nat
this✝ :
  ∀ (i : Nat) (hi : i ≤ n) (acc : List (Fin n)),
    fins.finsAux (Nat.succ n) i ⋯ (map Fin.castSucc acc ++ [Fin.last n]) =
      map Fin.castSucc (fins.finsAux n i hi acc) ++ [Fin.last n]
this :
  fins.finsAux (Nat.succ n) n ⋯ (map Fin.castSucc [] ++ [Fin.last n]) =
    map Fin.castSucc (fins.finsAux n n ⋯ []) ++ [Fin.last n]
⊢ fins.finsAux (Nat.succ n) n ⋯ [{ val := n, isLt := ⋯ }] = map Fin.castSucc (fins.finsAux n n ⋯ []) ++ [Fin.last n]
no goals