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Re: Backus-Naur Form for CIF

Peter Murray-Rust wrote

> [NB I 
> have never felt happy about the *semantics* of dot and query - it certainly 
> used to be possible to interpret them so that CIF processors were required 
> the to expand ? in DDLs  into default values in data files.]

The 'Guide to CIF for Authors' makes it clear that '?' is used when the
value is not known (i.e. it may or may not be the default).  '.' is
interepreted as the default (if a default exists).  

In my understanding '.' is also used to indicate that the value is
irrelevant in the current context in which case substituting the default
value should not cause any problem.  However, '?' should never be
substituted by the default.

It is not easy to find where these conventions are defined and I would
recommend that they be carefully and explicitly defined on Brian's
revamped website.

Dr.I.David Brown,  Professor Emeritus
Brockhouse Institute for Materials Research, 
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773

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