∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [Std.TransCmp cmp]
  {l : List (α × β)} {k k' fallback : α},
  cmp k k' = Ordering.eq →
    List.Pairwise (fun a b => ¬cmp a.1 b.1 = Ordering.eq) l →
      k ∈ List.map Prod.fst l → (Std.DTreeMap.Const.insertMany t l).getKeyD k' fallback = k