</p>
    </div>
    <div id="cautions-regarding-formulas">
    <h2>Cautions Regarding Formulas</h2>
    


<p>The use of <code>|</code> to define groups is tricky because (a) <code><a href="https://rdrr.io/r/stats/model.frame.html" class="external-link">stats::model.frame()</a></code>
doesn't handle this sort of thing and (b) <code>|</code> is also used for logical or.  The
current algorithm for handling this will turn the first  occurrence of <code>|</code> into an attempt
to condition, so logical or canno