valid expression. Else, returns
            a type convertible to the first element in the sequence.
          </p>
<p>
            <span class="bold"><strong>Precondition</strong></span>: <code class="computeroutput"><a class="link" href="empty.html" title="empty"><code class="computeroutput"><span class="identifier">empty</span></code></a><span class="special">(</span><span class="identifier">seq</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">false</span></code>