n><span class="va">d1</span> <span class="op">+</span> <span class="va">d2</span>     <span class="co"># same as c(d1, d2)</span></span></span>
<span class="r-out co"><span class="r-pr">#&gt;</span> |1, (1L 2L), 2, (1L 2L)|</span>
<span class="r-in"><span><span class="va">d2</span> <span class="op">+</span> <span class="va">d1</span>     <span class="co"># same as c(d2, d1)</span></span></span>
<span class="r-out co"><span class="r-pr">#&gt;</span> |2, (1L 2L), 1, (1L 2L)|</span>
<span class="r-in"><span></