PDL isn't familiar to me, and reading a few different discussions of it imply that it might not be worth learning in the first place. However, _Complete_ says you should attempt to turn it into English comments anyway, so I've decided to attempt to write my comments and unit tests first, then write all the code.
The problem so far is that I'm planning to do this recursively, and recursion doesn't lend itself easily to English. Plus I've got the logic of the subs mixed up in the body. A little reorganization should help here.
Oh, and I've got the Win32 installer and _Programming Ruby_ download. Once I get home later tonight, I'll start farting around with it. Should be fun.