∀ {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)} {k : α},
  (Std.DTreeMap.filterMap f t).get? k = (t.get? k).bind (f k)