∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap α β cmp} {γ : α → Type w} [Std.TransCmp cmp]
  {f : (a : α) → β a → γ a}, (Std.DTreeMap.filterMap (fun k v => some (f k v)) t).Equiv (Std.DTreeMap.map f t)