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Re: Meaning of _category.mandatory_code


>I see no technical reason why a future DDL could not provide a vocabulary with which CIF dictionaries could implement a hierarchical namespace such as David suggested.  It would, however, render all the CIF software on the planet obsolete -- a heavy cost that should be carefully weighed before going that route.  
>

CIF dictionaries written using DDL2 (macromolecular dictionaries) use 
save frames, but those written using DDL1 (small molecule dictionaries) 
do not.  Save frames are not used in any CIFs.  Thus at the moment the 
only software that needs to read save frames is that designed to read 
DDL2 and CIF2 dictionaries (i.e. CIF dictionaries written in DDL2)..  

Work has, however, started on preparing DDL3, a dictionary definition 
language that will be used to write CIF3 dictionaries, and these, like 
CIF2 dictionaries, will use save frames.  The main reason for 
introducing DDL3 is to remove the incompatiblitlies between DDL1 and 
DDL2 dictionaries.  This will be done by making sure that CIF3 
dictionaries can be used to read and validate any existing well-formed 
CIF regardless of which dictionary was used to write it.  This will be 
particularly useful for those working on the border between small and 
macromolecules.  The development of a new DDL provides an opportunity to 
introduce a number of advanced features such as a heirarchical attribute 
inheritance and methods, but since the programs that will read CIF3 
dictionaries will have to be able to handle save frames, it would add 
little additional overhead to allow save frames to be used in the CIF3s 
themselves.  Of course, current software is designed to read files 
written in either CIF1 or CIF2 (depending on which of the two standards 
it follows) and there is no reason why these standards should not 
continue to be used as long as they are useful.  However, CIFs that wish 
to exploit the features available in CIF3 would need to use software 
written for CIF3, but this software would also be able to read existing 
CIF1s and CIF2s, allowing CIF3 programs to access the whole CIF 
archive.  As CIF3 software becomes available the advantages in working 
in CIF3 will become apparent.

This email is designed to give a little context to my earlier remarks.  
It does not however address the problem that John raised, namely that 
the specification on how save frames are to be used is not very 
explicit.  This does need to be tightened up.

David Brown

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fn:I.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
tel;fax:+905 521 2773
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