Re: [ddlm-group] Finalizing DDLm

I have not had any time to respond to David�s original email or the
subsequent discussions. However I have discussed it with Syd.

DDLm defines loopable categories strictly and sub-categories of those have
strictly enforced outer joins (given we have ONLY considered sub-categories
that are List). This is how we overcome the split versus non-split versions
of atom_site and atom_site_aniso loops. List is strictly looped, Set is
strictly non-looped � contrary to your reading, and possibly Syd�s original
text. He has since re-read what he wrote and clarified the ambiguity. I will
send you that re-write shortly.

We had not considered a SET category being a sub-category of a List
category, but if it is allowed then it would not be an outer-join as you
suggested in your previous contribution but a relational Cartesian product
(which is very different).

The DDLm specification has that List category data MUST appear in a loop
(irrespective of how many rows there are), and SET categories are strictly
singular (non-looped data). The formal specification will formally remain
that way.

HOWEVER the IUCr is free to �extend� the specification of the DDLm for its
own internal and private use, so long as you appreciate that the FORMAL
published specification of what Syd and I have created can�t include it.

You might explain as to why you feel you have trouble with

loop_
�_atom_site.label
�_atom_site.frac_x
�_atom_site.frac_y
�_atom_site.frac_z
�Cu 0 0 0

And yet

�_atom_site.label �Cu
�_atom_site.frac_x �0.
�_atom_site.frac_y �0.
�_atom_site.frac_z �0.

Is so much more obvious? Given that people understand what the loop is, I
can't see what they would gain from the unrolled version (apart from
confusion). The real danger is those less experienced who DON'T read a
dictionary and read the latter form may be encouraged to replicate it when
there is more that 1 atom (thus corrupting the CIF structure).

However these are just personal observations, and if the IUCr wants to
qualify the use of DDLm with its own tweaks there is nothing stopping them
from doing so.

>>>>
>>>> At 10:35 AM -0500 3/11/10, David Brown wrote:
>>>>>
>>>>> Dear Colleagues,
>>>>>
>>>>> I assume that we are essentially finished in resolving syntax
>>>>> problems, but in that discussion some items were identified as being
>>>>> related to DDLm rather than syntax, so before we settle into serious
>>>>> dictionary writing we need to understand the DDLm rules.
>>>>>
>>>>> One item that I believe was raised under this heading was whether,
>>>>> if a loop contained a single set of items, it was necessary to
>>>>> formally include this in a loop structure. �If this is deemed to be
>>>>> necessary, then there has to be some way of identifying the items
>>>>> that must appear in a loop. �The presence in the dictionary of a
>>>>> _category_key.* item would seem to flag this, but it is applied at
>>>>> the level of the category rather than at the level of an individual
>>>>> item. �If the requirement that the loop structure must always be
>>>>> used, then all the items in the category must be loopable, i.e., the
>>>>> category cannot include items that would not normally be included in
>>>>> the loop, items for example that apply equally to all the listed
>>>>> items such as a scale factor that is the same for all the structure
>>>>> factors in a loop. �This seems to be workable, but I am not sure how
>>>>> the legacy CIFs would fit in, since categories may include some
>>>>> listable item and some non-listable items, and I am sure the
>>>>> listable items do not always appear in a loop if there is only one
>>>>> set of such items reported in the CIF.
>>>>>
>>>>> Is this something that can be clarified fairly easily? �It has an
>>>>> important bearing on how the CIF dictionaries are written.
>>>>>
>>>>> David
>>>>>
>>>>> Attachment converted: Macintosh HD:idbrown 55.vcf (TEXT/ttxt) (0046DFC7)
>>>>> _______________________________________________
>>>>> ddlm-group mailing list
>>>>> [email protected]
>>>>> http://scripts.iucr.org/mailman/listinfo/ddlm-group
>>>>
>>>>
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>>>> =====================================================
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>>>> � �Dowling College, Kramer Science Center, KSC 121
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cheers

Nick

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