The interest rate per period (as a decimal, must be ≥ 0).
The number of periods (must be a positive integer).
The payment amount per period (must be ≥ 0). Defaults to 1 if not provided.
The starting principal amount (must be ≥ 0). Defaults to 0 if not provided.
Class representing an annuity-immediate. Pays a fixed amount at the end of each period for
nperiods.