/* $DOC$
   $NAME$
      ft_NetPV()
   $CATEGORY$
      Math
   $ONELINER$
      Calculate net present value
   $SYNTAX$
      ft_NetPV( <nInitialInvestment>, <nInterestRate>, <aCashFlow> ;
                [, <nNoOfCashFlows> ] ) -> nNetPV
   $ARGUMENTS$
      <nInitialInvestment> is the amount of cash invested for purposes
      of generating the cash flows.

      <nInterestRate> is the annual interest rate used to discount
      expected cash flows (10.5% = 10.5, not .105).

      <aCashFlow> is an array of the expected cash receipts each year.

      <nNoOfCashFlows> is the number of years cash flows are expected
      (optional, Len( aCashFlow ) ).
   $RETURNS$
      The difference between the initial investment and the discounted
      cash flow in dollars.
   $DESCRIPTION$
      This function calculates the net present value, the difference
      between the cost of an initial investment and the present value
      of the expected cash flow(s) from the investment.  The present
      value of the expected cashflow(s) is calculated at the specified
      interest rate, which is often referred to as the "cost of capital".

      This function can be used to evaluate alternative investments.
      The larger the NPV, the more profitable the investment.  See
      also the FutureValue and PresentValue for further explanations.
      The formula to calculate the net present value is:

      NetPresentValue := SUM( CashFlow[ i ] / ( ( 1 + InterestRate ) ^ i ) )
      FOR i := 1 TO NoOfCashFlows
         //
      NEXT
   $EXAMPLES$
      ? ft_NetPV( 10000, 10, { 10000, 15000, 16000, 17000 } )
   $END$
 */