∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap α β cmp} {γ : α → Type w} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp] {f : (a : α) → β a → Option (γ a)},
  (Std.DTreeMap.filterMap f t).size = t.size ↔ ∀ (a : α) (h : a ∈ t), (f a (t.get a h)).isSome = true