Copying a snippet into HTML is where a documented language stops being the
real one. Each block on the page is a program here with its recorded output
beside it, and check.sh is what says the page is still true after a change.
Everything here is checked against the compiler rather than against plan.org:
the design documents describe a language larger than the one that runs, and a
page that documented the plan would mislead the first person to try it.
