write a function that will calculate the taxable capital gain or loss for each sale of a stock. Because the sale of stock may include units purchased at different times for different prices, this is not a trivial task. Furthermore, there is a 50% discount applied to capital gains where the units of stock have been held for at least 12 months (we consider this to be 365 calendar days). To identify which units of stock have been sold, the Australian Tax Office (ATO) uses the 'first in, first out' principle.