title: Stylus not allowing basic subtraction
body: <p>Noticed an issue when using semantic grids stylus. </p>
  
  <pre><code>*width total-width * ((((gutter-width + column-width ) * x) - gutter-width) / _gridsystem-width)-correction
  </code></pre>
  
  <p>is processing to something along the lines(read different numbers)</p>
  
  <pre><code>  *width: 89.5833333333333% -correction;
  </code></pre>
  
  <p>So, I decided to performan an experiment. When running <code>stylus -i</code> I found subtraction there was also not operating (at all)</p>
  
  <p>To verify I wrote this in my file</p>
  
  <pre><code>x=10
  y=20
  .something
      font-size y-x
  </code></pre>
  
  <p>which process to</p>
  
  <pre><code>.something {
    font-size: y-x;
  }
  </code></pre>
  
  <p>Am I crazy? Did I interpret operators incorrectly with stylus?
  Pending the results here I may open a ticket on github. I did find another person with an issue around percentages that highlights the <code>-</code> operator not functioning as expected. However, this doesn't seem related to my issue. </p>
  
  <p>Tldr: basic math without units doesn't seem to be working</p>