ass="external-link">sum</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/NA.html" class="external-link">is.na</a></span><span class="op">(</span><span class="va">course</span><span class="op">$</span><span class="va">number</span><span class="op">)</span><span class="op">)</span></span>
<span><span class="co">#&gt; [1] 0</span></span>
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