// Internal function to turn maps into its negative variant.
// It prefixes the keys with `n` and makes the value negative.
@function negativify-map($map) {
  $result: ();
  @each $key, $value in $map {
    @if $key != 0 {
      $result: map-merge($result, ("n" + $key: (-$value)));
    }
  }
  @return $result;
}