/// Swaps values at indexes `$a` and `$b` from `$list`
/// @access private
/// @param {List} $list - list to update
/// @param {Number} $a - index of first element
/// @param {Number} $b - index of second element
/// @return {List}
@function _swap($list, $a, $b) {
  @if abs($a) > length($list) or abs($b) > length($list) {
    @return $list;
  }

  $tmp: nth($list, $a);
  $list: set-nth($list, $a, nth($list, $b));
  $list: set-nth($list, $b, $tmp);
  @return $list;
}