number power(number, number)
Raises the first argument to the power of the second argument, returning the result. If the calculation does not result in a real number, then NaN is returned.
Examples:
power(-1, 0.5)
returns NaN.
if (prin>0 and dur>0 and rate>0, prin*rate/(1-power(1+rate, -dur)), 0)
returns a compounded interest payment value given a non-zero principal (prin), duration (dur) and periodic interest rate (rate).