simp
v t : VarName
X : DefName
xs : List VarName
h1 : v ∉ xs
⊢ def_ X (List.map (fun x => if v = x then t else x) xs) = def_ X xs
v t : VarName
X : DefName
xs : List VarName
h1 : v ∉ xs
⊢ List.map (fun x => if v = x then t else x) xs = xs