exact c1_right
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : t ∉ xs
a3 : t ∉ binders
a1 : v ∈ xs
a2 : (v = v → v ∈ binders) → v = t
c1_right : v ∉ binders
⊢ v ∉ binders
no goals