∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap α β cmp} [Std.TransCmp cmp] {k k' : α} {v : β},
  List.find? (fun x => cmp x.1 k == Ordering.eq) t.toList = some (k', v) ↔ t.getKey? k = some k' ∧ t[k]? = some v