title: Cats EitherT.collectRight can't find Alternative[Future]
body: <p>I'm trying to find an example for Cats <code>EitherT.collectRight</code>. I've a <code>EitherT[Future, String, Event]</code> and when I do <code>collectRight,</code> I get </p>
  
  <pre><code>Error:(79, 18) could not find implicit value for parameter FA: cats.Alternative[scala.concurrent.Future]
              elem.collectRight
  </code></pre>
  
  <p>I've an implicit <code>ExecutionContext</code> in scope, so <a href="https://github.com/typelevel/cats/blob/master/docs/src/main/tut/faq.md#future-instances" rel="nofollow noreferrer">this</a> FAQ doesn't apply.</p>