∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.ExtDTreeMap α β cmp} {γ : α → Type w}
  [inst : Std.TransCmp cmp] [inst_1 : Std.LawfulEqCmp cmp] {f : (a : α) → β a → Option (γ a)} {k : α},
  (Std.ExtDTreeMap.filterMap f t).getKey? k = (t.getKey? k).pfilter fun x h' => (f x (t.get x ⋯)).isSome