decoders writing control-1 chars?

[Ilya N. Golubev]

On 2005/11/28 02:10:19 +0 Vin Shelton committed in "stable"
`release-21-4' branch this change.

2005-04-05  Aidan Kehoe  <kehoea at parhasard.net>

	* mule-ccl.c (ccl_driver): Calculate the charset and position code
	for control-1 characters properly, both when reading multibyte
	characters and writing them. 

This changes what `write-multibyte-character' CCL instruction writes
when passed a representation of `control-1' charset character.  So the
change is obviously visible to lisp programmer.  Still it is not noted
in `NEWS' file.

`control-1' emchars are just like any others, and coding systems
decoding data to these emchars certainly make sense.  How do we
implement such a coding systems in CCL?  Assume that CCL program have
prepared generic emchar representation for `write-multibyte-character'
in its registers, and this emchar may be or not be in `control-1'
charset.  How do we have it write `control-1' emchar out after that
change?

Another issue is that emchar value being written out for `(control-1
CODE)' chars, 0 <= CODE < 32, is negative, which makes little sense.

Please resolve the issue.  Perhaps you decide to revert the change.
Or perhaps you devise and recommend the workaround.  Then please note
the change in <Lisp and internal changes> section of `NEWS' with
stating how to work around that, how to write `control-1' emchars out.