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Meaning of _category.mandatory_code

• Subject: Meaning of _category.mandatory_code
• From: James Hester <jrh@xxxxxxxxxxxx>
• Date: Fri, 27 Jan 2006 15:07:34 +0900

_category.mandatory_code is discussed in Volume G p 473 and in
mmcif_ddl_2.1.6.dic, which simply state "Whether or not the category
must be specified in a dictionary".  Vol G page 64 is more explicit and
partially contradictory: "whether or not this category must appear in a
data block based on this dictionary."

So the interpretation is clear for e.g. the space_group category of the
cif_sym dictionary: any CIF data block claiming to conform with this
dictionary must have at least one data item belonging to the space_group
category.  Likewise, the dictionary category in the core ddl2 dictionary
must appear, and it does, in the datablock outside the dictionary save
frames.

However, what about the _item_description category in the core ddl2
dictionary?  It has _category.mandatory_code set to 'yes'.  Taking the
p64 description, this is saying no more than that an attribute from the
_item_description category must appear somewhere in a DDL2 dictionary.
I'm not aware of any explicit description of save frame scoping in CIF
dictionaries, but I deduce that we are supposed to adopt a model
whereby, from the perspective of a single dictionary save frame, the
datablock consists precisely of the save frame plus the enclosing
datablock, that is, the enclosing datablock is to a save frame what a
global block is to subsequent datablocks in a STAR file.  In this
scoping model, we deduce that all save frame definitions must at the
very least include an _item_description attribute.  Unfortunately, this
cannot be the case, as some save frames are category definitions and so
an item_description is absent.  Either we should drop the mandatory
specification for _item_description, or improve the wording to allow
e.g. _category_description to substitute for _item_description.  

Comments? 

James.
-- 
_______________________________________________________________________
James Hester, ANBF                             KEK
e-mail: jrh@anbf2.kek.jp                       Oho 1-1
Phone: +81 298 64 7959                         Tsukuba, Ibaraki 305
  Fax: +81 298 64 7967                         Japan
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