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Re: T.61 -> Unicode conversion

Howard Chu wrote: 
Hallvard B Furuseth wrote:

Also,

I'm pretty sure that when I used X.500 and T.61 (10+ years ago?), at
least the T.61 Underline combining character could be combined with
other combining characters.  I don't know if Unicode allows that; the
strprep description of doing so at the end of Appendix A does not.

You're right, the notes I read state that the Underline can be combined with any other graphic character. I'll have to double-check how Unicode deals with this.

http://www.unicode.org/book/ch02.pdf page 25
The Unicode Standard does not restrict the number of combining characters that may follow a base character.

And from http://www.itscj.ipsj.or.jp/ISO-IR/103.pdf
The non-spacing character UNDERLINE shall never be used individually but always in combination with some other graphic character to represent the graphic rendition "underlined" for the associated character. The non-spacing character UNDERLINE can be used in combination with any graphic character of the repertoire, including an accented letter or an umlaut, or SPACE.

This note would imply that we need to add a check for this case to Appendix A: if the combining character is UNDERLINE and the next character is also a combining character, then the character after that is used as the base character and both of the combining characters are mapped and appended. E.g. "_'a" becomes "a_'"

--
  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc
  Symas: Premier OpenSource Development and Support