∀ {α : Type u} {instOrd : Ord α} {β : Type v} {t : Std.DTreeMap.Internal.Impl α fun x => β} [Std.TransOrd α],
  t.WF →
    ∀ {l : List (α × β)} {k k' fallback : α},
      compare k k' = Ordering.eq →
        List.Pairwise (fun a b => ¬compare a.1 b.1 = Ordering.eq) l →
          k ∈ List.map Prod.fst l → (↑(Std.DTreeMap.Internal.Impl.Const.insertMany! t l)).getKeyD k' fallback = k