have : sign (x.inv * x) = sign (1 : Permutation n) := x.inv_mul ▸ rfl
m n : Nat
x : Permutation n
⊢ sign (inv x) = sign x
m n : Nat
x : Permutation n
this : sign (inv x * x) = sign 1
⊢ sign (inv x) = sign x