∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α]
  [inst : Inhabited α], t.WF → ∀ {a : α} {b : β a}, (Std.DTreeMap.Internal.Impl.insert! a b t).getKey! a = a