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