class="op">=</span> <span class="cn">TRUE</span><span class="op">)</span>, <span class="st">"df.K"</span><span class="op">)</span></span></span>
<span class="r-out co"><span class="r-pr">#&gt;</span> [1] 7</span>
<span class="r-in"><span></span></span>
<span class="r-in"><span><span class="co"># There are two restrictions:</span></span></span>
<span class="r-in"><span><span class="fu"><a href="https://rdrr.io/r/base/attr.html" class="external-link">attr</a></span><span class="op">(</span><span class="fu"><a