@function _list-flatten($list) {
  $return-list: ();
  @each $item in $list {
    @if type-of($item) == list {
      $return-list: join($return-list, _list-flatten($item));
    } @else {
      $return-list: append($return-list, $item);
    }
  }
  @return $return-list;
}