subst a2_right
v : Var
X : String
vs : List Var
a1 : ∃ a ∈ vs, v ∈ a.boundVarSet
i : ℕ
a2_left : bound_ i ∈ vs
a2_right : v = bound_ i
⊢ v ∈ vs ∧ v.isBound
X : String
vs : List Var
i : ℕ
a2_left : bound_ i ∈ vs
a1 : ∃ a ∈ vs, bound_ i ∈ a.boundVarSet
⊢ bound_ i ∈ vs ∧ (bound_ i).isBound