That example is what got me to start learning J which I have always found to be unreadable, much prefer my array languages to have their non-ascii symbols. A few nights playing with J and learning was enough to not be completely lost and able to make some progress, but it is still a challenge. When it says it "describes and implementation of J" it is not kidding, it describes the implementation and goes no further. Both the article and the code stick to this sort of terse and very concise language.