Unicode String Models

[Eli Zaretskii via Unicode]

> Date: Wed, 12 Sep 2018 00:13:52 +0200
> Cc: unicode at unicode.org
> From: Hans Åberg via Unicode <unicode at unicode.org>
> 
> It might be useful to represent non-UTF-8 bytes as Unicode code points. One way might be to use a codepoint to indicate high bit set followed by the byte value with its high bit set to 0, that is, truncated into the ASCII range. For example, U+0080 looks like it is not in use, though I could not verify this.

You must use a codepoint that is not defined by Unicode, and never
will.  That is what Emacs does: it extends the Unicode codepoint space
beyond 0x10FFFF.