run only if <code class="code">work</code> or <code class="code">always</code>
    failed with an exception (typically, undoing user-visible state
    changes that would only make sense if the function completes
    correctly). For example:</p>

<pre class="codepre"><code class="code">      <span class="keyword">let</span> objfile = outputprefix ^ <span class="string">".cmo"</span> <span class="keyword">in</span>
      <span class="keyword">let</span> oc = open_out_bin objfile <span class="keyword">in</span