Note that we use string interpolation with {}</span></span></span>
<span class="r-in"><span>    <span class="fu">warn_up</span><span class="op">(</span><span class="st">"The lengths of x and y don't match: {nx} vs {ny}."</span><span class="op">)</span></span></span>
<span class="r-in"><span>  <span class="op">}</span></span></span>
<span class="r-in"><span></span></span>
<span class="r-in"><span>  <span class="va">x</span> <span class="op">+</span> <span class="va">y</span></span></span>
<span class="r-in">