title: Sass check if variable is passed to mixin
body: <p>Lets say I have a mixin with default values</p>
  
  <pre><code>$default: 1;
  
  @mixin test($p : $default) {
      @if () {
          // Do something if $p was 4passed?
      } @else {
          // Do something if $p was not passed?
      }
  
  }
  </code></pre>
  
  <p>If I was to call the mixin like so, two different was:</p>
  
  <pre><code>@include test(); // A
  @include test(1);  // B
  </code></pre>
  
  <p>Is there a way I can tell the difference between the default <strong>(A)</strong>, and the one that's overwriting the default <strong>(B)</strong>?</p>