title: LESS mixins with multiple arguments
body: <p>I have this little mixin set up:</p>
  
  <pre><code>.linear-gradient(@direction:top, @color1:#fff, @color2:#000) 
  {
      background-image: -webkit-linear-gradient(@direction, @color1, @color2);
  }
  </code></pre>
  
  <p>Then, in my <code>main.less</code> I'm trying something like this:</p>
  
  <pre><code>a { .linear-gradient(top, #999, #666); }
  </code></pre>
  
  <p>That works fine, by say I want to do something like this:</p>
  
  <pre><code>a { .linear-gradient(top,     , #666); }
  </code></pre>
  
  <p><strong>Now, the first color should default to its default mixin color. How do I do this? Is this even possible</strong></p>