constructor
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∉ binders
x : VarName
⊢ v = x ∧ x ∉ binders ↔ v = x
case mp
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∉ binders
x : VarName
⊢ v = x ∧ x ∉ binders → v = x

case mpr
v t : VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : v ∉ binders
x : VarName
⊢ v = x → v = x ∧ x ∉ binders