∀ {α : Type u} {instOrd : Ord α} {β : Type v} {t : Std.DTreeMap.Internal.Impl α fun x => β} [Std.TransOrd α]
  [inst : BEq α] [Std.LawfulBEqOrd α] (h : t.WF) {l : List (α × β)},
  List.Pairwise (fun a b => ¬compare a.1 b.1 = Ordering.eq) l →
    (∀ (a : α), Std.DTreeMap.Internal.Impl.contains a t = true → (List.map Prod.fst l).contains a = false) →
      (↑(Std.DTreeMap.Internal.Impl.Const.insertMany t l ⋯)).size = t.size + l.length