nt. Has to be <code>NULL</code>
if no references are specified.</p></dd>


<dt id="arg-reference">reference<a class="anchor" aria-label="anchor" href="#arg-reference"></a></dt>
<dd><p>A vector of reference levels in the following format:
If <code>contrasts</code> is not <code>NULL</code>, the desired reference level(s). This
must be a vector or list the same length as <code>contrasts</code>, and if not named,
it is assumed that the order is as specified in contrasts.</p></dd>


<dt id="arg-seed">seed<a clas