Trying to find may way

Usage of stopifnot() for data checks within functions.
Trying to find my way in R on how to organize specialized functions
with clearly defined parameters (e.g. FV.periodic) and functions where
the parameters arbitrage takes place (e.g. FV).
A first attempt at writing an IRR function in R, a complete failure

To be completely rewritten. I should find a moment and implement the
algo myself.