∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [inst : Std.TransOrd α]
  (h : t.WF) {k : α} {he : (Std.DTreeMap.Internal.Impl.erase k t ⋯).impl.isEmpty = false},
  ¬compare k (t.maxKey ⋯) = Ordering.eq → (Std.DTreeMap.Internal.Impl.erase k t ⋯).impl.maxKey he = t.maxKey ⋯