a></span><span class="op">(</span><span class="st">"John"</span>, <span class="st">"Paul"</span>, <span class="st">"George"</span><span class="op">)</span><span class="op">)</span><span class="op">)</span></span>
<span><span class="co">#&gt;       Ringo zguitarists</span></span>
<span><span class="co">#&gt; birds    24          54</span></span></code></pre></div>
<p>It is an error to aggregate over a margin and leave identically named
rows or columns. The following function call will fail, because it
aggreg