New data name _exptl_absorpt.special_details

[James H]

Dear Core DMG,

Antanas Vaitkus has discovered that _exptl_absorpt_special_details has
never been defined in an official dictionary although it appears over 1000
times in the Crystallographic Open Database. There is an obvious use case,
as shown in a "SHELX for neutrons" tutorial that contains an example using
this tag to record how absorption values for neutrons are calculated.
Antanas' description of the issue can be read at
https://github.com/COMCIFS/cif_core/issues/234 , and the slide show with
example of use of this tag is at
https://chemistry.harvard.edu/files/chemistry/files/tutorial_using_shelx_for_neutrons.pdf

Note that the dictionary already defines a
tag _exptl_absorpt_process_details, which is defined as

    Description of the absorption correction process applied to the
    measured intensities. A literature reference should be supplied
    for psi-scan or multi-scan techniques.

As Antanas notes, many of those 1000 files include values both for this
data name as well as _exptl_absorpt_special_details, meaning that this is
not a coding mistake.

Please comment on whether or not you agree with this data name being added
to the core dictionary. I believe the definition is self-evident, along the
lines of "Information not covered by other data names in the exptl_absorpt
category, for example, details of the calculation of the absorption
correction factor mu". Please note that silence means that you consent to
the data name being added!

Of course, we would rather not be playing catch-up with definitions, as we
are in this case, so I would encourage any of you who see a deficit in our
suite of definitions to take the lead in discussing adding them to the
dictionaries. I'd also note that commandeering a data name in the core CIF
namespace in this way is undesirable, as it could lead to complete
breakdown in our processes (such as they are) and lack of clarity on the
meaning of data names in a given CIF file. Fortunately this seems to be an
isolated case.

thanks,
James.
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