∀ {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 → γ} {k : α}, (Std.ExtTreeMap.map f t).contains k = true → t.contains k = true