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