XMI12 — Encoding of strings and characters in XMI documents

The document production rules for encoding of strings and characters (9.5.9.23
& 9.5.9.24) need to be clarified.

They need to say how to encode significant whitespace characters so that they
don't get mangled by XML document builders and parsers. Leading and trailing
whitespace are particularly worrisome. One possible solution is surround the
entire character or string value with matching quotes, and say that anything
outside of the quotes is significant.

They also need to say that characters and strings are encoded using ISO-10646
(as per 3.1.5).