public function assign($key, $value)
    {
        if ($this->$key === false) {
            $this->all[$key] = $value;
        } elseif (! is_array($this->$key)) {
            $this->all[$key] = array($this->$key, $value);
        } else {
            $this->all[$key][] = $value;
        }
    }