TODO

- Fix typechecking of union; as Nate Foster points out, the way it is done
  now is plain wrong (well, too weak) To make checking of disjoint union
  not annoy the hell out of people, need to support setminus for regexps

- better error message for "short split in concat"

- this needs to make aug_save fail:

  set /files/etc/apt/sources.list.d/bar/type deb
  set /files/etc/apt/sources.list.d/bar/uri http://server.com/
  set /files/etc/apt/sources.list.d/bar/distribution etch
  save

  The paths are missing a 1/ after the bar/ and do not conform to the schema

- completion of paths with spaces in augtool needs to be smarter about quoting
  spaces

- permutation operator: often, options for commands can be permuted in any
  order. Using '*' for that is a bit of a kludge since it allows giving the
  same option many times.

- sshd.aug is wrong: it does not express that Match must come at the end of
  the file and that anything after a match only applies to that
  block. That's specifically a problem if a new entry is added to the tree;
  that will show up in a Match block, when it should show up _before_ the
  first Match block

- Better POSIX compliance for fa_compile: in particular, support named
  character classes and allow /|a/ to mean 'epsilon or a'

- Sprinkle the words 'get' and 'put' into error messages from the lens
  typechecks to make it clearer which direction (and regexps) they are
  talking about