 1     0     0</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> 2:      b     2     0     1     0</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> 3:     NA     3     0     0     1</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> 4:     NA     4     0     0     1</span>
<span class="r-in"><span></span></span>
</code></pre></div>
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