∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α],
  t.WF →
    ∀ {k a : α},
      Std.DTreeMap.Internal.Impl.contains a (Std.DTreeMap.Internal.Impl.erase! k t) = true →
        Std.DTreeMap.Internal.Impl.contains a t = true