title: Compose futures by Scala Async
body: <p>There are futures f1, f2 and f3. If we need wait till both f1 <code>and</code> f2 complete <code>or</code> f3 does, i.e. the condition looks like <code>completed(f1) &amp; completed(f2) | completed(f3)</code>, how can we use <a href="https://github.com/scala/async" rel="nofollow">Scala Async</a> to compose it fluently?</p>