title: Parent Selector in Nested Stylus Mixins
body: <p>Is there a selector or available hack to add styles to parents through a mixin? It would only need to work in cases where there is a nested set up. For example, if I were to have:</p>
  
  <pre><code>#overbox
    width 100vw
    height 100vh
    position fixed
    top 0 
    left 0
    background-color rgba(90,90,90,.9)
    #contentbox
      width 480px
      height 320px
      background-color #e4e4e4
  </code></pre>
  
  <p>I have a mixin to vertically align an element, like so:</p>
  
  <pre><code>valign()
    position relative
    top 50%
    transform translateY(-50%)
  </code></pre>
  
  <p>Which works exactly like I'd expect, but I would like also to add <code>tranform-style preserve-3d</code> to the parent element, #overbox, to prevent border fuzziness, etc. I've had no success. I am looking for something that would work like:</p>
  
  <pre><code>valign()
    position relative
    top 50%
    transform translateY(-50%)
    &amp;:parent
      transform-style preserve-3d
  </code></pre>
  
  <p>Anyone know a solution/workaround? </p>