The ANNUITY function (annuity immediate) returns a numeric value that approximates the ratio of an annuity paid at the end
of each period for the number of periods specified by argument-2 to an initial investment of one. Interest is earned at the
rate specified by argument-1 and is applied at the end of the period, before the payment. The type of this function is numeric.

- Argument-1 must be class numeric.
- The value of argument-1 must be greater than or equal to zero.
- Argument-2 must be a positive integer.

- When the value of argument-1 is zero, the value of the function is the approximation of:
1 / argument-2

- When the value of argument-1 is not zero, the value of the function is the approximation of:
argument-1 / (1 - (1 + argument-1) ** (- argument-2))

- Floating-point format is used for numeric non-integer results.