Discussion paper on allowing redefinition of datanames in CIF

[James Hester]

Dear COMCIFS,

In the process of preparing the DDLm version of the modulated structures
dictionary, it came to my attention that this dictionary nowhere defines a
dataname to hold the new model structure factor.  This turns out to lead to
a more fundamental issue involving most dictionaries, and requires some
sort of resolution from this group. I have written a discussion paper to
get us started which I've placed at
https://github.com/COMCIFS/comcifs.github.io/blob/master/changing_meanings_discussion_paper.md

The key proposal is to allow dictionaries to redefine datanames as long as
the old meaning can be viewed as a special case of the new meaning. A new
dataname to flag redefinitions is included.

It would be great if any of you with an interest in describing experiments
not covered by core CIF (magnetism, modulation, twinning, multipole,
polarisation, powder, electron microscopy,...) could consider these issues
and provide feedback, corrections and improvements.  Note that this is not
particularly CIF-specific.

Once the opinion of this group is clear, we can delegate the detailed work
to the DDLm-group after which a formal proposal would return here for
approval.

James.
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