To help you get the most from the text and keep track of what's happening, we've used a number of conventions throughout the book.
Boxes with a warning icon like this one hold important, not-to-be forgotten information that is directly relevant to the surrounding text.
The pencil icon indicates notes, tips, hints, tricks, and asides to the current discussion.
As for styles in the text:
We use a monofont type with no highlighting for most code examples. We use bold to emphasize code that's particularly important in the present context.