/**
  * Checks if value exists.
  *
  * - NOTE: Allows deep get. See $this->get().
  *
  * @param string $key
  * @param string|array $subkeys
  * @return bool
  */
 protected function exists($key, $subkeys = null)
 {
     $exists = isset($this->data[$key]);
     if ($exists && !empty($subkeys)) {
         $exists = UTILS::array_splat((array) $this->data[$key], $subkeys);
     }
     return $exists;
 }