ng expressions very simple, that feature is not really useful and can be achieved using standard subsetting notation in R:</p>
<div class="sourceCode"><pre class="sourceCode r"><code class="sourceCode r">x &lt;-<span class="st"> </span><span class="kw">lm</span>(mpg <span class="op">~</span><span class="st"> </span><span class="kw">factor</span>(cyl) <span class="op">*</span><span class="st"> </span>am <span class="op">+</span><span class="st"> </span>hp <span class="op">+</span><span class="st"> </span>wt,