code: public function merge($newItems)
      {
          if ($newItems instanceof self) {
              $newItems = $newItems->toArray();
          }
  
          if (!is_array($newItems)) {
              throw new \InvalidArgumentException('Combine must be a Dictionary or an array');
          }
  
          $items = array_merge($this->storage, $newItems);
  
          return new static($this->keyType, $this->valType, $items);
      }
docs: {@inheritdoc}