According to Excel's own helpfiles, it appears that ISPMT is there for
compatibility with Lotus.
ISPMT: Calculates the interest paid during a specific period of an
investment. This function is provided for compatibility with Lotus
1-2-3.
IPMT: Returns the interest payment for a given period for an investment
based on periodic, constant payments and a constant interest rate. For a
more complete description of the arguments in IPMT and for more
information about annuity functions, see PV.
