∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.ExtTreeSet α cmp} [inst : Std.TransCmp cmp] [inst_1 : BEq α]
  [Std.LawfulBEqCmp cmp] {l : List α} {k : α}, k ∈ t.insertMany l ↔ k ∈ t ∨ l.contains k = true