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CIF specification: reserved prefixes

  • To: comcifs@iucr.org
  • Subject: CIF specification: reserved prefixes
  • From: Brian McMahon <bm@iucr.org>
  • Date: Tue, 28 Oct 2003 12:06:15 +0000
Dear Colleagues

While working through the content of International Tables Volume G, I have
come upon two statements that are in flat contradiction. In the CIF
specification, regarding the use of registered prefixes to reserve a
namespace of data items for local use, it is stated:

     \P 12. There is no syntactic property identifying such a reserved
            prefix, so that software validating or otherwise handling
            such local data names must scan the entire registry and
            match registered prefixes against the indicated components
            of data names. Note that reserved prefixes may themselves
            contain underscore characters, so a maximal matching search
            must be made.

while in chapter 3.1, "General considerations when defining a CIF data
item", I find I have written Reserved prefixes

     To guarantee that locally devised data names may be placed without
     name conflict in interchange data files, authors may register a
     reserved character string for their sole use. As with the special
     prefix _[local]_ discussed in the preceding section, the author's
     reserved prefix is simply an underscore-bounded string within the data
     name (i.e. it may not itself include an underscore character).

I can in fact see no useful purpose in permitting an underscore *within* a
registered prefix - it simply complicates the task of the parser. At this
stage no prefixes have been registered with an embedded underscore. I
therefore propose to amend paragraph 12 of the CIF working spec, replacing
the final sentence by

     "Note that reserved prefixes may not themselves contain underscore

Please let me have your opinions on this as quickly as possible. I shall
continue to work on the Volume on the assumption that no objections will be
raised to this amendment.


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