∀ {α : Type u} {cmp : α → α → Ordering} [inst : Std.TransCmp cmp] {l : List α} {k k' fallback : α},
  cmp k k' = Ordering.eq →
    List.Pairwise (fun a b => ¬cmp a b = Ordering.eq) l →
      k ∈ l → (Std.ExtDTreeMap.Const.unitOfList l cmp).getKeyD k' fallback = k