title: less.js guards and conditionals
body: <p>I have the following less mixin:</p>
  
  <p><code>@myColor = #123456;</code></p>
  
  <p><code>.mixin(@a) when (@a = @myColor){
        // do something
      }</code></p>
  
  <p>This however throws this error: Unable to perform comparison</p>
  
  <p>Why is that?</p>