∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  {f : α → β → Bool} {k : α},
  (Std.ExtDTreeMap.filter f t).getKey? k = (t.getKey? k).pfilter fun x h' => f x (Std.ExtDTreeMap.Const.get t x ⋯)