Use of @at-root to append an element to the front of a class

atRootAppend.css

.btn {
  height: 18px;
  background: green;
  line-height: 12px;
}
a.btn {
  line-height: 18px;
}

atRootAppend.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

.btn {
    height:18px;
    background:green;
    line-height:12px;
    
    //If it's attached to a link, make the line height bigger
    @at-root a#{&} {
      line-height:18px;
    }
}