rcases i with ⟨i,hi⟩
m n : Nat
i j : Fin n
h : i < j
⊢ sign (transpos i j) = true
case mk
m n : Nat
j : Fin n
i : Nat
hi : i < n
h : { val := i, isLt := hi } < j
⊢ sign (transpos { val := i, isLt := hi } j) = true