pan class="hljs-value">: true;</span>
  <span class="hljs-at_rule">@<span class="hljs-keyword">if</span><span class="hljs-preprocessor"> type-of</span>($box) != <span class="hljs-string">"string"</span> {</span>
    <span class="hljs-at_rule">@<span class="hljs-keyword">warn</span> <span class="hljs-string">"box: #{$box} is not a string"</span>;</span>
    $valid<span class="hljs-value">: false;</span>
  }
  <span class="hljs-at_rule">@<span class="hljs-keyword">if</span> $valid ==<span class="hljs-preproce