To do:

- Document: mention that REX parsing R.E. doesn't break down tags into
  components (such as attribute name/value pairs), so they are written
  as is.  Length is assumed to be equivalent to the number of characters,
  which can mess up wordwrap if there are embedded linefeeds or carriage
  returns.

- Try on Windows.
- Need some shorter alternatives to the really long option names.

- Add command-line options to select which type of operation to perform:
  check integrity (input should be identical to output, both by
  concatenating token list, and after converting to tree)
- Allow multiple configuration files.  Then one file can be used as the
  base, with later files as modifiers to that.
- Allow *block and *inline special element names that are used in preference
  to *DEFAULT? These could be useful to avoid having to declare options for
  elements other than what type they are.
  (Precedence: explicit, *block|*inline, *DOCUMENT)
- Allow indenting of comments, PIs?