2706 - start work on tangle
2707 - the simplest possible narrative

I've been stuck, unable to decide between building the narrative
and building the tools to tangle the narrative into working code.
What got me unstuck was putting the core data structure first, with
forking paths to read() and eval(). That works better than the
tokenize-parenthesize-parse-eval pipeline I've always had.

Other things I'm not sure how to designate:
  append somewhere without having to name fragments (like leo's @other)
  code vs tests
  tests that only pass for now but will eventually be invalid
2708

Tentative syntax for the three cases in the last commit message.
2709 - tentative syntax for hyperlinks

[] just says, 'try to find a target for this link'. Headings are obvious
candidates. What about others?