ng functions with posynomial arguments:
<tt class="docutils literal"><span class="pre">sum</span></tt>, <tt class="docutils literal"><span class="pre">cumsum</span></tt>, <tt class="docutils literal"><span class="pre">mean</span></tt>, <tt class="docutils literal"><span class="pre">prod</span></tt>, <tt class="docutils literal"><span class="pre">cumprod</span></tt>.</li>
</ul>
<p>A valid generalized posynomial expression is</p>
<ul class="simple">
<li>a valid posynomial;</li>
<li>the sum of two or more gene