simp at h2
v u : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∉ binders
h2 :
  BoolFormula.def_ X (List.map (fun v => decide (v ∈ binders)) xs) =
    BoolFormula.def_ X (List.map (fun v => decide (v ∈ binders)) (List.map (fun x => if v = x then u else x) xs))
⊢ v ∈ xs → u ∉ binders
v u : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∉ binders
h2 :
  List.map (fun v => decide (v ∈ binders)) xs =
    List.map ((fun v => decide (v ∈ binders)) ∘ fun x => if v = x then u else x) xs
⊢ v ∈ xs → u ∉ binders