∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  [Std.LawfulEqCmp cmp] [inst_2 : Inhabited β] {f : α → β → Bool} {k : α},
  Std.ExtDTreeMap.Const.get! (Std.ExtDTreeMap.filter f t) k =
    (Option.filter (f k) (Std.ExtDTreeMap.Const.get? t k)).get!