title: How does the Promise.coroutine support generator as the yieldable value?
body: <p><code>Promise.coroutine</code> supports <code>Promise</code> as the yieldable value type. And via the <code>addYieldHandler(function handler)</code>, <code>Promise.coroutine</code> can also support any types that retuning result only once. But how could I write a yieldHandler that can handle a <code>generator</code> type like <a href="https://github.com/visionmedia/co" rel="noreferrer">co</a> does?</p>