∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.ExtDTreeMap α β cmp} [inst : Std.TransCmp cmp]
  [inst_1 : Std.LawfulEqCmp cmp] {k a : α} [inst_2 : Inhabited (β a)],
  (t.erase k).get! a = if cmp k a = Ordering.eq then default else t.get! a