function getBillState($billId, $multicall = false)
    {
        if (!is_int($billId)) {
            throw new InvalidArgumentException('billId = ' . print_r($billId, true));
        }

        if ($multicall) {
            return $this->execute(ucfirst(__FUNCTION__), array($billId), $this->structHandler('Bill'));
        }
        return Structures\Bill::fromArray($this->execute(ucfirst(__FUNCTION__), array($billId)));
    }