1.  Reporting line numbers for errors.
2.  Checking classes of code blocks.
3.  Tidying up omissions found by running against example lesson.

1.  Merging @twitwi's expand/collapse.
2.  Replacing text with icon for show/hide.
3.  Rewriting example to show other possibilities.

The file is not *needed* for anything, because the information is in
the Git history.  It's just easier to access when it's committed to a
file in the repo than it is to ask curious folks to clone the repo and
run ‘git shortlog’, etc.

This file was removed in 2e05819f (Naming layouts and inclusions more
clearly, 2016-04-19).

GitHub repositories know the repository they were forked from (sort of
like the usual 'origin' remote), but I'm not aware of them having
other remote-like references.

Besides dropping the "other remotes" bit, soften the 'origin' wording.
I'm not sure if GitHub uses 'origin' remotes or not to track the
parent repository, and the implementation details matter less than the
fact that there *is* a reference of some sort to the parent.