Naviagtion icons take the form of a button bar at the top of
each page, and (usually) a single button at the bottom.
The button bar looks like:
The icons and their meanings are:
An unhighlighted button with a border indicates the page is not
yet available, for example:
Move to the beginning of the previous lecture.
Move to the previous page in this lecture.
Move up one level, returning to the Course home page.
Move to the next page in this lecture.
Move to the beginning of the next lecture.
Toggle color/nocolor versions of this page (if it exists).
A button with no border indicates the page does not exist, for example