@function z-index($key1, $key2: null) {
  $map: $z-indexes;
  $found-index: null;

  @if (map-has-key($map, $key1)) {
    $found-index: index(map-keys($map), $key1);

    @if ($key2) {
      $map: map-get($map, $key1);
      $found-index: index($map, $key2);
    }
  }

  @return $found-index;
}

@function z($key1, $key2: null) {
  @return z-index($key1, $key2);
}