subst c2_left
v t x y : VarName
binders : Finset VarName
h1 : t ≠ x ∧ t ≠ y
c1 : ¬(v = x ∧ x ∉ binders)
a1 : t = x ∨ t = t
c2_left : v = y
c2_right : y ∉ binders
⊢ v ∉ binders
v t x : VarName
binders : Finset VarName
c1 : ¬(v = x ∧ x ∉ binders)
a1 : t = x ∨ t = t
h1 : t ≠ x ∧ t ≠ v
c2_right : v ∉ binders
⊢ v ∉ binders