Describe if-then-else, while, until
Revert back to original semantics for sub-expression capture

- The intention was to have a tool akin to "%s", but for lists, and
  with the added twist that an expression that doesn't yield anything
  still produces an empty list.  The result was unconvincing and
  required a complex analysis to even work (we do count with having
  the analysis, but that should be an optimization, not an absolute
  requirement).

  With if-then-else available, all the current uses of [X] in examples
  become unnecessary anyway--this was only used for [X] (if, else),
  which was ingenious, but ultimately not very readable.

  So get back to the original thing where [X] is a literal form--a
  pure pusher, and X is evaluated in sub-expression context.
Dump in-progress documentation

- Much of this is essentially thinking alound, some of it is updating
  examples for new syntax, some of it experimenting with syntax
  without formal backing (namely the <X>?eq bits).