N/2456
L2/02-210
ISO
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION
ORGANISATION INTERNATIONAL DE NORMALISATIONISO/IEC JTC 1/SC 2/WG 2 Universal Multiple-Octet Coded Character Set
(U C S)
Title: Removal of two Variant Sequences Source: U.S. National Body Status: Liaison Action: For consideration by JTC1/SC2/WG2 in Dublin Date: 2002-05-07 Summary
The UTC has agreed to remove the variant sequences <2278, FE00> and <2279, FE00> from Unicode 3.2.1, and requests that WG2 should do the same in a corrigendum or amendment to ISO/IEC 10646 to ensure the continued synchronization of the two standards.
The U.S. National Body agrees with and supports this proposal.
Background
At the last minute in the release of 3.2, the Unicode consortium was notified of a problem with the variation selectors. The UTC had earlier agreed (after much discussion), that it must restrict the application of VSs to non-decomposables, non-combining marks (there are a host of problems if VSs are allowed to apply to other characters). As it turns out, there was a mistake in two cases.
Because it was too late to make a change, and to allow for coordination with WG2, the Unicode editorial committee simply added the following note to Unicode 3.2 explaining the situation:
Note: Just before publication, an inconsistency was discovered between the above principles and the standardization of the two variant sequences <2278, FE00> and <2279, FE00> because U+2278 and U+2279 are in fact decomposable characters. Those variant sequences denote glyph variants of these mathematical symbols with a vertical line instead of a slanted line as the diacritic to indicate the negation.
The sequence <2278, FE00> is canonically equivalent to <2276, 0338, FE00>, and the sequence <2279, FE00> is canonically equivalent to <2277, 0338, FE00>. So that these equivalent sequences are given equivalent rendering treatment, the use of U+FE00 would have to be interpreted—exceptionally—as defining a variant appearance for the entire sequence.
Because a combining vertical line overlay, U+20D2 COMBINING LONG VERTICAL LINE OVERLAY, is also available in the Standard, an alternate way of explicitly indicating these particular variants already exists. That alternative mechanism is a safer and more stable way to indicate the distinction, as the inherent complications in allowing variation selectors to follow combining marks may require future corrective action to remove the exceptional variant sequences <2278, FE00> and <2279, FE00> from the table.
[See http://www.unicode.org/reports/tr28/#13_7_variation_selectors]
The removal of these two variant sequences does not affect the ability of Unicode/10646 to express the variation, since there was already a way to do that using U+20D2. (Had we all noticed this earlier, the variant sequences never would have been added.)