vel; `"factor" = custom(a,b,c..)` will alter each
means by an amount of a for the first, b for the second, etc. Finally
`"factor" = Rexpression("R code")` will apply R code to all levels of
the factors, altering the base mean.</p>
    </div>

    <div id="ref-usage">
    <div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">slope</span><span class="op">(</span><span class="va">s</span><span class="op">)</span></span>
<span></span>
<span><span class="fu">extent</span><span class="op"