cases c1
case pos
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 : v = x ∧ x ∉ binders
⊢ v ∉ binders
case pos.intro
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
left✝ : v = x
right✝ : x ∉ binders
⊢ v ∉ binders