simp only [Var.openList]
j : ℕ
zs : List String
i : ℕ
⊢ (Var.openList j (List.map free_ zs) (bound_ i)).freeVarSet ⊆ (bound_ i).freeVarSet ∪ (List.map free_ zs).toFinset
j : ℕ
zs : List String
i : ℕ
⊢ (if i < j then bound_ i
      else
        if x : i - j < (List.map free_ zs).length then (List.map free_ zs)[i - j]
        else bound_ (i - j - (List.map free_ zs).length + j)).freeVarSet ⊆
    (bound_ i).freeVarSet ∪ (List.map free_ zs).toFinset