code: public function merge($items)
      {
          if ($items instanceof CollectionInterface) {
              $items = $items->toArray();
          }
  
          if (!is_array($items)) {
              throw new InvalidArgumentException('Merge must be given array or Collection');
          }
  
          $this->validateItems($items, $this->type);
          $newItems = array_merge($this->items, $items);
  
          $col = new static ($this->type);
          $col->setItemsFromTrustedSource($newItems);
  
          return $col;
      }
docs: {@inheritdoc}