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Re: Meaning of _category.mandatory_code

Just one point of clarification.  item_description is a category in DDL2 
which contains two items, _item_description.name and 
_item_description.description.  Clearly a dictionary save frame would be 
incomplete without the item's name being present 
(_item_description.name) hence it is mandatory.  As James points out 
however, these two items do not appear in the save frames that describe 
categories.  However, to correct James on a minor point, the name of the 
category is not given in _category_description (this is the category 
equivalent of _item_description.description) but in the item 
_category.id.  _category.id is also described as mandatory but only 
appears in the category save frames, not the item save frames.  Thus the 
mandatory 'yes' should only apply to _category.id in save frames that 
describe the attributes of a category, and _item_description.name should 
only be mandatory in save frames that describe the attributes of an 
item.  It is not clear to me that there is an easy way to resolve this 
problem.  (_category.id is not to be confused with _item.category_id 
which identifies the category that an item belongs to - as if this were 
not confusing enough already)

However one interprets the star and cif descriptions, it seems clear to 
me that what is intended is that a save frame should be read in 
conjunction with surrounding datablock (i.e., ignoring only the other 
save frames in the data block).  Since programs that access CIF2 
dictionaries (CIF dictionaries written using DDL2) need to be able to 
handle save frames, there should be no objection to introducing save 
frames into CIFs that conform to yet-to-be-written CIF3 dictionaries 
(i.e., CIF dictionaries written in the proposed DDL3).  This would have 
the advantage of allowing people to describe more than one experiment in 
the same CIF - all the items common to both experiments would be in the 
datablock while the description of the two experiments would be in two 
different save frames.  However this would only make sense if a save 
frame is to be read in the context of the surrounding datablock.  Our 
present procedure of including several related datablocks in the same 
file is unsatisfactory because datablocks are explicitly stated to be 
independent and there is no satisfactory way of linking them.

David Brown
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org:McMaster University;Brockhouse Institute for Materials Research
adr:;;King St. W;Hamilton;Ontario;L8S 4M1;Canada
email;internet:idbrown@mcmaster.ca
title:Professor Emeritus
tel;work:+905 525 9140 x 24710
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