∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.TreeSet.Raw α cmp} [Std.TransCmp cmp] [inst : BEq α]
  [Std.LawfulBEqCmp cmp] [inst_2 : Inhabited α],
  t.WF → ∀ {l : List α} {k : α}, k ∉ t → l.contains k = false → (t.insertMany l).get! k = default