This page uses the basic format developed in the Code_Mockups page,
then adds support for code folding:
Code folding is a feature of some text editors,
source code editors, and IDEs that allows the user
to selectively hide and display - "fold" - sections
of a currently-edited file as a part of routine edit operations.
This allows the user to manage large amounts of text
while viewing only those subsections of the text
that are specifically relevant at any given time.
There are at least two plausible approaches to doing code folding in HTML.
The ones we're currently experimenting with use: