Haskell, improved

I showed my Haskell program to a co-worker today. As we were looking at it, we noticed some possible improvements. (Surprising, since neither of us are Haskell programmers.)

I tried the changes this evening, and they do indeed work. We've simplified the program and made the logic clearer. I also expanded the program to handle a list of payment values, and not assume that each payment is the same value. It was quite easy to do -- much easier than similar changes to a C++ or C# program.