SCSS mixin creating a stacked block element

_stacked.scss

////
// Makes a div appear to be a stacked like 3 sheets of paper with each
// sheet offset by a given amount.
//
// Example: @include stacked(5px);
@mixin stacked($offset) {
  border: 1px solid #494949;
  
  &:after, &:before {
    background: inherit;
    border: 1px solid #494949;
    bottom: ($offset + 2) * -1;
    content: '';
    height: 100%;
    width: 100%;
    left: $offset;
    position: absolute;
    z-index: -10;
  }

  &:before {
      bottom: $offset * -2 - 3;
      left: $offset * 2 + 1;
      position: absolute;
      z-index: -20;
  }
}