hetics can be
specified:</p>
<ul>
<li><p><code>weight</code>: select a weighting variable</p></li>
<li>
<p><code>x</code>: select variables to add to formula</p>
<ul>
<li>declared as <code>x = product(var2, var1, ...)</code>
</li>
</ul>
</li>
<li>
<p><code>alpha</code>: add an alpha transparency to the selected
variable</p>
<ul>
<li>unless the variable is called in <code>x</code>, it will be added to
the formula in the first position</li>
</ul>
</li>
<li>
<p><code>fill</code>: select a variable to be filled