I couldn't figure out how to email the author and I don't have twitter, but I found a few mistakes in the "More power" section that tripped me up a bit as I followed along, so just posting here in case it helps anyone else:

The end of the second last code block should be:

`data.instructions.forEach(...args.map(parseInstruction));`

And the full definition of `data.instructions` that demonstrates the use of this modification would be something like:

`[["rotate", ["rotate", ["drawLine", { x: 0, y: 0 }, { x: 1, y: 1 }], 90], 45]]`