subst a2
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : t ∉ xs
x : VarName
a1 : x ∈ xs
a3 : t ∉ binders
c1 : v = x → x ∈ binders
a2 : x = t
⊢ v ∉ binders
v : VarName
X : DefName
xs : List VarName
binders : Finset VarName
x : VarName
a1 : x ∈ xs
c1 : v = x → x ∈ binders
h1 : x ∉ xs
a3 : x ∉ binders
⊢ v ∉ binders