∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.ExtDTreeMap α (fun x => Unit) cmp} [inst : Std.TransCmp cmp]
  {l : List α} {k k' fallback : α},
  cmp k k' = Ordering.eq →
    k ∉ t →
      List.Pairwise (fun a b => ¬cmp a b = Ordering.eq) l →
        k ∈ l → (Std.ExtDTreeMap.Const.insertManyIfNewUnit t l).getKeyD k' fallback = k