e and should be used sparingly.  Finally their
result can only be used once in a pipeline (else they will try to clobber their own result).</p>
    </div>

    <div id="ref-usage">
    <div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">non_sql_node</span><span class="op">(</span></span>
<span>  <span class="va">source</span>,</span>
<span>  <span class="va">...</span>,</span>
<span>  f_db <span class="op">=</span> <span class="cn">NULL</span>,</span>
<span>  f_df <span class="op"