∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.DTreeMap α (fun x => Unit) cmp} [Std.TransCmp cmp] [inst : BEq α]
  [Std.LawfulBEqCmp cmp] {l : List α} {k fallback : α},
  k ∉ t → l.contains k = false → (Std.DTreeMap.Const.insertManyIfNewUnit t l).getKeyD k fallback = fallback