∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.TreeSet.Raw α cmp} [Std.TransCmp cmp] [inst : BEq α]
  [Std.LawfulBEqCmp cmp], t.WF → ∀ {l : List α} {k : α}, (k ∈ t.insertMany l) = (k ∈ t ∨ l.contains k = true)