∀ {α : Type u} {β : Type v} {γ : Type w} {cmp : α → α → Ordering} {t : Std.ExtTreeMap α β cmp} [inst : Std.TransCmp cmp]
  {f : α → β → Option γ} {k k' : α} {fallback : γ},
  t.getKey? k = some k' → (Std.ExtTreeMap.filterMap f t).getD k fallback = (t[k]?.bind (f k')).getD fallback