ial">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span></code></a>
            is called by <code class="computeroutput"><span class="identifier">as</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
            to determine validity; if it returns false, the generator fails. Subsequent
            extraction is performed by calling <a class="link" href="../../../advanced/customize/attribute_as.html" title="Atomically