subst a2
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∈ binders
x : VarName
a✝ : x ∈ xs
a2 : v = x
a3 : x ∉ binders
⊢ t = x
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∈ binders
a✝ : v ∈ xs
a3 : v ∉ binders
⊢ t = v