public function add($key, $value)
    {
        $storage = $this->storage;
        $storage[$key] = $value;

        return new static($this->keyType, $this->valType, $storage);
    }
public function add($key, $value)
    {
        $storage = $this->storage;
        $storage[$key] = $value;

        return new static($this->keyType, $this->valType, $storage);
    }
{@inheritdoc}