Find the present value of an income stream that has a negative flow of
$100 per year for 3 years, a positive flow of $200 in the 4th year,
and a positive flow of $300 per year in Years 5 thru 8.  The
appropriate discount rate is 4% for each of the first 3 years and 5%
for each of the later years.  Thus, a cash flow accruing in Year 8
should be discounted at 5% for some years and 4% in other years.  All
payments occur at year-end.
     Note:  The solution should be somewhere between $500 and $1400.