Fail if we would hold negative shares of asset after completing this
        order.

def validate(self,
                 asset,
                 amount,
                 portfolio,
                 algo_datetime,
                 algo_current_data):
        """
        Fail if we would hold negative shares of asset after completing this
        order.
        """
        if portfolio.positions[asset].amount + amount < 0:
            self.handle_violation(asset, amount, algo_datetime)