rcases k with ⟨k⟩ | ⟨k⟩
m n : Nat
x : Permutation n
k : Int
h : sign x = false
⊢ sign (x ^ k) = false
case ofNat
m n : Nat
x : Permutation n
h : sign x = false
k : Nat
⊢ sign (x ^ Int.ofNat k) = false

case negSucc
m n : Nat
x : Permutation n
h : sign x = false
k : Nat
⊢ sign (x ^ Int.negSucc k) = false