subst c1_left
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
c1_left : v = x
c1_right : x ∉ binders
⊢ v ∉ binders
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : t ∉ xs
a1 : v ∈ xs
a2 : (v = v → v ∈ binders) → v = t
c1_right : v ∉ binders
⊢ v ∉ binders