simp at h2
v u : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∉ binders
a1 : v ∈ xs
h2 : decide (v ∈ binders) = ((fun v => decide (v ∈ binders)) ∘ fun x => if v = x then u else x) v
⊢ u ∉ binders
v u : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∉ binders
a1 : v ∈ xs
h2 : v ∈ binders ↔ u ∈ binders
⊢ u ∉ binders