∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.TreeMap α Unit cmp} [Std.TransCmp cmp] [inst : BEq α]
  [Std.LawfulBEqCmp cmp] [inst_2 : Inhabited α] {l : List α} {k : α},
  k ∉ t → l.contains k = false → (t.insertManyIfNewUnit l).getKey! k = default