(</span><span class="st">"A"</span>,<span class="st">"B"</span>,<span class="st">"C"</span>,<span class="st">"D"</span><span class="op">)</span><span class="op">)</span></span></span>
<span class="r-out co"><span class="r-pr">#&gt;</span>    A  B  C  D</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> A NA  1  2  3</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> B  1 NA  4  5</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> C  2  4 NA  6</span>
<span class="r-out co"><