code: public function sortKeysWith(callable $comparator)
      {
          $items = $this->items;
  
          uksort($items, $comparator);
  
          return $this->createFrom($items);
      }
docs: Returns a bag with the keys sorted with the `$comparator`.
  
  @param callable $comparator Function given `($keyA, $keyB)`
  
  @return static