public function toString($indent = null)
    {
        $indent = ($indent !== null)
                ? $this->getWhitespace($indent)
                : $this->getIndent();

        $return    = $this->prefix;
        $keys      = array_keys($this->container);
        $lastIndex = end($keys);
        foreach ($this->container as $index => $item) {
            $item = array_merge($this->defaultParams, (array) $item);
            $return .= vsprintf($this->wrapper, $item);
            if ($index !== $lastIndex) {
                $return .= $this->separator;
            }
        }
        $return .= $this->postfix;

        return preg_replace("/(\r\n?|\n)[ ]*/", '$1' . $indent, $return);
    }