∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [Std.TransCmp cmp]
  [inst : Inhabited β] {k a : α},
  Std.DTreeMap.Const.get! (t.erase k) a = if cmp k a = Ordering.eq then default else Std.DTreeMap.Const.get! t a