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