rewrite [List.mem_cons, not_or] at h
case cons
m n✝ n : Nat
i j : Fin n
l : List (Fin n)
IH : ¬i ∈ l → sign (List.foldr (fun j x => swap x i j) 1 l) = (List.length l % 2 == 1)
h : ¬i ∈ j :: l
⊢ sign (swap (List.foldr (fun j x => swap x i j) 1 l) i j) = (Nat.succ (List.length l) % 2 == 1)
case cons
m n✝ n : Nat
i j : Fin n
l : List (Fin n)
IH : ¬i ∈ l → sign (List.foldr (fun j x => swap x i j) 1 l) = (List.length l % 2 == 1)
h : ¬i = j ∧ ¬i ∈ l
⊢ sign (swap (List.foldr (fun j x => swap x i j) 1 l) i j) = (Nat.succ (List.length l) % 2 == 1)