intro x a1 a2 a3
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : t ∉ xs
⊢ ∀ x ∈ xs, ((v = x → x ∈ binders) → x = t) → t ∉ binders → v ∉ binders
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : t ∉ xs
x : VarName
a1 : x ∈ xs
a2 : (v = x → x ∈ binders) → x = t
a3 : t ∉ binders
⊢ v ∉ binders