∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.ExtDTreeMap α (fun x => Unit) cmp} [inst : Std.TransCmp cmp]
  [inst_1 : Inhabited α] {l : List α} {k : α},
  k ∈ t → (Std.ExtDTreeMap.Const.insertManyIfNewUnit t l).getKey! k = t.getKey! k