intro i hi acc
n : Nat
⊢ ∀ (i : Nat) (hi : i ≤ n) (acc : List (Fin n)), length (fins.finsAux n i hi acc) = length acc + i
n i : Nat
hi : i ≤ n
acc : List (Fin n)
⊢ length (fins.finsAux n i hi acc) = length acc + i