∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.ExtTreeMap α 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 → (t.insertManyIfNewUnit l).getKeyD k' fallback = k