title: LESS: Mixin with optional parameter
body: <p>I have a LESS mixin with this code:</p>
  
  <pre><code>.generated_width (@margins:40px)
  {
    -webkit-width:calc(~"100% - " @margins );
       -moz-width:calc(~"100% - " @margins );
            width:calc(~"100% - " @margins );
  }
  </code></pre>
  
  <p>But sometimes I need to OPTIONALLY specify a % width different, for example "50%". Obviously, I could create another specific, but different, mixin with "50%" replacing "100%", but I prefer to pass this value as a parameter.</p>
  
  <p>Is there a way to edit my mixin in order to OPTIONALLY accept a parameter that overrides default value of "100%" ? In other words, sometime the call could be:</p>
  
  <pre><code>.generated_width (40px)
  </code></pre>
  
  <p>Others</p>
  
  <pre><code>.generated_width (50%,40px)
  </code></pre>