e>. However, it is OK to for <code>next ()</code> to return <code>None</code> when <code>is_empty</code> is false. This corresponds to the case where there is more work to be done, but it is not schedulable until some already scheduled work is finished.</p></td></tr></table><code>}</code></dt><dd><p>abstraction for generating jobs</p></dd></dl><dl><dt class="spec value" id="val-chain"><a href="#val-chain" class="anchor"></a><code><span class="keyword">val</span> chain : <span><span>(<span class="type-var">'