case intro c1_left c1_right =>
  subst c1_left
  exact c1_right
v t x y : VarName
binders : Finset VarName
h1 : t ≠ x ∧ t ≠ y
c2 : ¬(v = y ∧ y ∉ binders)
a1 : t = t ∨ t = y
c1_left : v = x
c1_right : x ∉ binders
⊢ v ∉ binders
no goals