title: Can I extend external scss rule without including it in the output?
body: <p>GIVEN that there is existing scss file that defines rules like <code>.btn {..}</code> or <code>.btn-primary</code>...</p>
  
  <p>I WANT to declare my own rules by extending existing rules</p>
  
  <pre><code>.my-button {
    @extend .btn
    @extend .btn-primary
  }
  </code></pre>
  
  <p><strong>without actually including the <code>.btn</code> and <code>.btn-primary</code> classes in my generated css file?</strong></p>
  
  <p>Normally I need to <code>@import exiting.scss</code>, but that includes all the rules from the file in my css  output.</p>