∀ {α : Type u} {β : Type v} {γ : Type w} {cmp : α → α → Ordering} {t : Std.ExtTreeMap α β cmp} [inst : Std.TransCmp cmp]
  [Std.LawfulEqCmp cmp] {f : α → β → Option γ} {k : α}, (Std.ExtTreeMap.filterMap f t)[k]? = t[k]?.bind fun x => f k x