title: LESS Css failed at calculate js variable?
body: <p>I'm new to <code>LESS CSS</code>. When I use below <code>html</code> and <code>LESS css</code> codes the output comes error.</p>
  
  <p>Please fix this problem.</p>
  
  <p><strong>index.html</strong></p>
  
  <pre><code>&lt;!DOCTYPE HTML&gt;
  &lt;html&gt;
  &lt;head&gt;
  &lt;title&gt;LESS CSS&lt;/title&gt;
  &lt;script type="text/javascript" src="../_s/jquery.js"&gt;&lt;/script&gt;
  &lt;link rel="stylesheet/less" type="text/css" href="test.less" /&gt;
  &lt;script src="less.js" type="text/javascript"&gt;&lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  
  &lt;div id="box"&gt;&lt;/div&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p><strong>test.less</strong></p>
  
  <pre><code>@color : #2AA4CF;
  @clientHeight: ~`$.trim( $(window).height() ) + 'px'`;
  #box
  {
      position: absolute;
      background-color: darken(@color, 10%);
      width: 300px;
      height: @clientHeight - 100;
  }
  </code></pre>
  
  <p>Output is Error</p>
  
  <pre><code>Object # has no method 'operate'
  in test.less 
  at e.Operation.eval (http://localhost/frames/006/less.js:9:11151)
  at Object.e.Expression.eval (http://localhost/frames/006/less.js:9:3533)
  at Object.e.Value.eval (http://localhost/frames/006/less.js:9:18173)
  at e.Rule.eval (http://localhost/frames/006/less.js:9:12727)
  at Object.e.Ruleset.eval (http://localhost/frames/006/less.js:9:13904)
  at Object.e.Ruleset.eval (http://localhost/frames/006/less.js:9:13904)
  at Object.toCSS (http://localhost/frames/006/less.js:8:10291)
  at http://localhost/frames/006/less.js:9:20740
  at http://localhost/frames/006/less.js:8:1157
  at Object.p.parse (http://localhost/frames/006/less.js:8:10899)
  </code></pre>
  
  <p>Please give me a solution...</p>