Check if the given order book is valid.

        :param currency: Major currency name in lowercase.
        :type currency: str | unicode
        :raise InvalidCurrencyError: If an invalid major currency is given.

def _validate_currency(self, currency):
        """Check if the given order book is valid.

        :param currency: Major currency name in lowercase.
        :type currency: str | unicode
        :raise InvalidCurrencyError: If an invalid major currency is given.
        """
        if currency not in self.major_currencies:
            raise InvalidCurrencyError(
                'Invalid major currency \'{}\'. Choose from {}.'
                .format(currency, tuple(self.major_currencies))
            )