suffices ∀ (y : Permutation n), (l.foldr _ y)[i.1] = y[i.1] by
  rewrite [this 1]; exact get_one i.1
case cons
n : Nat
i j : Fin n
l : List (Fin n)
h : ¬i = j ∧ ¬i ∈ l
⊢ (List.foldr (fun j_1 x => swap x j j_1) 1 l)[↑i] = i
case cons
n : Nat
i j : Fin n
l : List (Fin n)
h : ¬i = j ∧ ¬i ∈ l
⊢ ∀ (y : Permutation n), (List.foldr (fun j_1 x => swap x j j_1) y l)[↑i] = y[↑i]