r">Key</span><span class="special">&gt;(</span><span class="identifier">seq</span><span class="special">);</span>
</pre>
<p>
            <span class="bold"><strong>Return type</strong></span>: Convertible to <code class="computeroutput"><span class="keyword">bool</span></code>.
          </p>
<p>
            <span class="bold"><strong>Semantics</strong></span>: Evaluates to <code class="computeroutput"><span class="keyword">true</span></code> if the sequence contains an element
            associated with K