case _ i =>
  simp only [Var.boundVarSet] at a2
  simp at a2
  cases a2
  case _ a2_left a2_right =>
    subst a2_right
    simp only [isBound]
    simp
    exact a2_left
v : Var
X : String
vs : List Var
a1 : ∃ a ∈ vs, v ∈ a.boundVarSet
i : ℕ
a2 : bound_ i ∈ vs ∧ v ∈ (bound_ i).boundVarSet
⊢ v ∈ vs ∧ v.isBound
no goals