∀ {α : Type u} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α fun x => Unit} [Std.TransOrd α] [inst : BEq α]
  [Std.LawfulBEqOrd α],
  t.WF →
    ∀ {l : List α},
      List.Pairwise (fun a b => ¬compare a b = Ordering.eq) l →
        (∀ a ∈ t, l.contains a = false) →
          (↑(Std.DTreeMap.Internal.Impl.Const.insertManyIfNewUnit! t l)).size = t.size + l.length