n class="p">(</span><span class="n">f</span><span class="p">)(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">xs</span><span class="p">...)</span> <span class="o">==</span> <span class="n">f</span><span class="p">(</span><span class="n">y</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">xs</span><span class="p">...));</span>
</pre></div>
</div>
</div>
<div class="section" id="requirements"