I've just sent the appended mail to the XML working group.
Misha
~~~
Background info
----------------
ISO 10646 supports a large number of 64K planes. Unicode 2.0 supports
the first seventeen planes: plane 0, aka the Basic Multilingual Plane
(BMP), and the next sixteen planes (planes 1-16). Encoding schemes
vary in the planes they support:
UCS-4 All planes
UTF-8 All planes
UTF-16 Planes 0-16
UCS-2 Plane 0
UTF-16 encodes all BMP characters identically to UCS-2. Thus the only
difference between UTF-16 and UCS-2 is that UCS-2 cannot encode planes
1-16.
Problem
--------
The XML spec is self-contradictory in regard to ISO 10646 planes beyond
the BMP. On the one hand:
L1. Section 4.3.3 includes the sentence: "It is recognised that ...
the use of additional planes ... may be required.".
L2. Section 4.3.3 includes the sentence: "The values ... UTF-16 ...
should be used for the various encodings ... of ... ISO/IEC 10646
...".
On the other hand:
R1. The SGML Declaration for XML is based on UCS-2, ie admits only the
BMP.
R2. UTF-16 is, I think, mentioned nowhere else in the XML spec.
Proposed solution
------------------
S1. Both in the "SGML Declaration for XML" and in the box labelled
"Scope Document", amend the BASESET to reference IR-177 (ie UCS-4)
rather than IR-176 (ie UCS-2). This will bring the XML spec into
line with the HTML 4.0 spec.
S2. In both these places, amend the DESCSET accordingly: In the "SGML
Declaration for XML", change the "65376" to "2147483488". In the
box labelled "Scope Document", amend the "65536" to " 2147483648".
The SGML Declaration for HTML 4.0 has "2147483486" rather than
"2147483488" (see the least significant digit), but I'll seek to
have that changed.
S3. Replace *all* other instances of "UCS-2" with "UTF-16".
S4. Add a note explaining that UCS-2 is a subset of UTF-16.
------------------------------------------------------------------------
Misha Wolf Email: misha.wolf@reuters.com 85 Fleet Street
Standards Manager Voice: +44 171 542 6722 London EC4P 4AJ
Reuters Limited Fax : +44 171 542 8314 UK
------------------------------------------------------------------------
Eleventh International Unicode Conference, Sep 2-5 1997, www.unicode.org
------------------------------------------------------------------------
Any views expressed in this message are those of the individual sender,
except where the sender specifically states them to be the views of
Reuters Ltd.
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