∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap α β cmp} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp] {a : α} [inst_1 : Inhabited (β a)], a ∈ t → t.get? a = some (t.get! a)