congr!
v t : VarName
X : DefName
xs : List VarName
h1 : t ∉ xs
⊢ def_ X (List.map (fun x => if t = x then v else x) (List.map (fun x => if v = x then t else x) xs)) = def_ X xs
case h.e'_2
v t : VarName
X : DefName
xs : List VarName
h1 : t ∉ xs
⊢ List.map (fun x => if t = x then v else x) (List.map (fun x => if v = x then t else x) xs) = xs