span><span class="va">b</span> <span class="op">&lt;-</span> <span class="fu">graph_from_literal</span><span class="op">(</span><span class="va">X</span> <span class="op">-</span> <span class="op">+</span><span class="va">Y</span>, <span class="va">Ur</span> <span class="op">-</span> <span class="op">+</span><span class="va">X</span>, <span class="va">Ur</span> <span class="op">-</span> <span class="op">+</span><span class="va">Y</span><span class="op">)</span></span></span>
<span class="r-in"><span><span c