protected function getAppearingVatRates(array $lines)
    {
        $vatRates = array();
        foreach ($lines as $line) {
            if (isset($line[Tag::VatRate])) {
                $vatRate = sprintf('%.1f', $line[Tag::VatRate]);
                if (isset($vatRates[$vatRate])) {
                    $vatRates[$vatRate]++;
                } else {
                    $vatRates[$vatRate] = 1;
                }
            }
        }
        return $vatRates;
    }