∀ {α : Type u} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α fun x => Unit} [Std.TransOrd α] [inst : Inhabited α],
  t.WF →
    ∀ {l : List α} {k : α},
      k ∈ t → (↑(Std.DTreeMap.Internal.Impl.Const.insertManyIfNewUnit! t l)).getKey! k = t.getKey! k