This is an exercise intended to satisfy Khan Academy's take-home test. If you this by googling their take-home test, shame on you! Do your own homework.

Cf. SPEC.txt for full details.

Simply run, "make test" to run all of the tests.

I'd really love to hook some visualization up to Population(). I'm imaginging prefuse-style spring model ball-and-edge graphs showing how users are connected to one another, and color coding users as the feature spreads among them. I suspect doing a good job on this front end would take about the same time I've put into this back end. So if I do it, it'll just be for fun.

I'd also really like to make support for multiple feature infections explicit throughout. It seems really artificial that you only be able to select across the graph one feature at a time, and it would be nice to use Population.randomize() to create big, statistically plausible user test sets that put people into multiple overlapping or orthogonal conditions.