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RE: Backus-Naur descriptions for STAR and CIF

  • Subject: RE: Backus-Naur descriptions for STAR and CIF
  • From: "Bollinger, John Clayton" <jobollin@xxxxxxxxxxx>
  • Date: Wed, 17 May 2000 15:23:07 +0100 (BST)

Brian McMahon wrote:
> The specific restrictions were:
>    No use of global_
>    No use of save_ frames
>    Loops no deeper than level 1
>    Maximum line length (80 characters)
>    Maximum length of datanames and datablock codes
> The last restriction has been relaxed; there is no formal 
> length restriction
> on datanames and block codes, but in practice they are 
> restrained by the
> persistent 80-column record length. The loop level 
> restriction was specified
> in the Hall et al paper; I suspect the global_ and save_ 
> condiitons were not
> raised simply because the authors saw no point in drawing attention to
> such features for the sole purpose of excluding them.


Thanks for the clarification.  What about designating the end of looped
data with "stop_"?  Without nested loops that is unnecessary, of
course, but it does not appear to actually be excluded.  (And
Spadaccini's CIF BNF provides for it.)

And how about putting the above list in the CIF section of the IUCr
site?  As I observed earlier, parts of it do not seem to be documented
there at all.

> So where does this leave us? Do we need one BNF, for STAR, 
> and then handle
> all other cases as special, with different exceptions 
> hard-coded? Or do we
> need multiple BNFs (STAR, CIF, dictionaries - even DDL1 and DDL2
> dictionaries)? There was some debate about this on COMCIFS a 
> while back,
> and we didn't really reach a conclusion. I think the answer will be a
> pragmatic one, based on the needs of the software community. 
> So this is an
> opportunity to poll the list on just that point.

Well, an appropriate question would seem to be: "what is the purpose
of having a BNF representation?"  I would argue that the principal
reason is to have a concrete, formal, precise definition of the
language(s) for reference purposes.  Once we have a final BNF
representation for STAR, then, do we need additional BNFs for
restricted dialects of STAR?  Is it less precise or concrete if we say
that a CIF, for example, is a STAR file that also satisfies certain
(specified) structural and syntactic constraints?  In the cases of CIF,
DDL1, and DDL2, I don't think so.  In my opinion, the nature of the
relevant constraints is such that specific BNF representations of those
languages wouldn't help us much more than the STAR BNF and the relevant
set of restrictions.


John Bollinger
Indiana University
Molecular Structure Center


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