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[ddlm-group] Treatment of CIF2 unicode characters with CIF1equivalents

  • To: ddlm-group <ddlm-group@iucr.org>
  • Subject: [ddlm-group] Treatment of CIF2 unicode characters with CIF1equivalents
  • From: James Hester <jamesrhester@gmail.com>
  • Date: Mon, 24 Apr 2017 09:45:42 +1000
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Dear DDLm-group (aka COMCIFS technical committee)

There has been some lively discussion on the cif-developers mailing list of late which you may review at http://www.iucr.org/__data/iucr/lists/cif-developers/ .

One issue raised was what to do about CIF2 datavalues that contained unicode characters that have equivalent ASCII sequences described by the CIF markup conventions (e.g. Greek characters).

According to section 2.2.7.4.13 - 17 of International Tables Vol G, by default Greek and some other non-ASCII characters can be represented in text datavalues using a backslash notation <backslash><ascii character>, e.g. \a is alpha.   Different markup conventions are possible on a per-dictionary or per-definition basis. In CIF2, these characters can be represented natively, but legacy CIF applications presented with a datavalue containing non-ASCII values may not be prepared to typeset or present them appropriately.  On the other hand, it would seem inefficient to define separate Unicode-aware datanames for every text value simply to avoid legacy problems.

Proposal: add the following paragraph to Vol G section 2.2.7.4. Note that "meets the requirements of paragraph 2.2.7.4.13" means that this paragraph only applies in those cases for which the CIF1 markup conventions would apply. 

(2.2.7.4.18) Whenever an application is required to convert a datavalue from a CIF2 datafile containing code points outside the ASCII range to a datavalue containing only ASCII codepoints, the appropriate markup as per paragraphs 2.2.7.4.13-16 should be substituted, provided that the relevant definition meets the requirements of paragraph 2.2.7.4.13. If no markup is defined for the Unicode code point, no CIF1 equivalent value exists and application behaviour is undefined.

Please comment.

James.

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