title: How can I create a calc mixin to pass as an expression to generate tags?
body: <p>I am working on a sass stylesheet in which I wish to use the <code>calc</code> element to dynamically size some content. As the <code>calc</code> element has not been standardized, I need to target <code>calc()</code>, <code>-moz-calc()</code>, and <code>-webkit-calc()</code>.</p>
  
  <p>Is there a way for me to create a mixin or function that I can pass an expression to so that it will generate the required tags that can then be set as a <code>width</code> or <code>height</code>?</p>