title: LESS variable usage in mixin paramaters
body: <p>I was hoping to create LESS mixin for border-radius that allows for two parameters to be set as defaults by two other parameters in the mixin declaration. The following is an example, that does not work, but resembles what I'm trying to achieve:</p>
  
  <pre><code>.border-radius-ext (@topleft: 0, @topright: 0, @bottomright: @topleft, @bottomleft: @topright) {
      border-top-left-radius: @topleft;
      border-top-right-radius: @topright;
      border-bottom-right-radius: @bottomright;
      border-bottom-left-radius: @bottomleft;
  }
  </code></pre>
  
  <p>I realize I could have all the values set to 0 to begin with. That's not what I'm after. I was hoping that if the mixin were used like so:</p>
  
  <pre><code>blockquote {
      .border-radius-ext(3px, 5px);
  }
  </code></pre>
  
  <p>The mixin would output:</p>
  
  <pre><code>blockquote {
      border-top-left-radius: 3px;
      border-top-right-radius: 5px;
      border-bottom-right-radius: 3px;
      border-bottom-left-radius: 5px;
  }
  </code></pre>
  
  <p>...while still allowing the defaults for <code>@bottomright</code> and <code>@bottomleft</code> to be overridden if the mixin were used like so:</p>
  
  <pre><code>blockquote {
      .border-radius-ext(3px, 5px, 7px, 2px);
  }
  </code></pre>
  
  <p>In that instance, the output would be:</p>
  
  <pre><code>blockquote {
      border-top-left-radius: 3px;
      border-top-right-radius: 5px;
      border-bottom-right-radius: 7px;
      border-bottom-left-radius: 2px;
  }
  </code></pre>
  
  <p>Is this not possible with LESS or am I just doing it wrong?</p>