∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {γ : Type w} {t : Std.DTreeMap α (fun x => β) cmp}
  [Std.TransCmp cmp] {f : α → β → Option γ},
  (Std.DTreeMap.filterMap f t).size = t.size ↔
    ∀ (k : α) (h : k ∈ t), (f (t.getKey k h) (Std.DTreeMap.Const.get t k h)).isSome = true