title: How to use Compass filter mixin?
body: <p>I'm trying to use filter mixin this way in SCSS:</p>
  
  <pre><code>a {
    @include filter(grayscale(100%));
  }
  </code></pre>
  
  <p>But when I compile, I got this error:</p>
  
  <pre><code>Undefined mixin 'filter'.
  </code></pre>
  
  <p>I'm using the latest version of this Gem with Rails framework.</p>
  
  <p><a href="http://compass-style.org/reference/compass/css3/filter/" rel="noreferrer">http://compass-style.org/reference/compass/css3/filter/</a></p>