public function addConfig(array $config, $environment = null)
    {
        $env = ($environment)? $environment : '_top';
        $this->values[$env] = (array_key_exists($env, $this->values))?
            array_replace_recursive($this->values[$env], $config)
            : $config
        ;
        $this->buildCache = null;
        return $this;
    }