code: public function map(callable $callback)
      {
          $items = [];
  
          foreach ($this->items as $key => $value) {
              $items[$key] = $callback($key, $value);
          }
  
          return $this->createFrom($items);
      }
docs: Applies the `$callable` to each _value_ in the bag and returns
  a new bag with the items returned by the function.
  
  Note: This differs from {@see array_map} in that the callback is passed `$key` first, then `$value`.
  
  @param callable $callback Function is passed `($key, $value)`
  
  @return static