Providing a definition for the deprecated _symmetry_cell_setting

[James Hester]

Dear Core DMG,

A sharp-eyed reader of the new DDLm core_CIF dictionary has noted that
_space_group.crystal_system has _symmetry_cell_setting as an alias, even
though _symmetry_cell_setting includes 'rhombohedral' as a possible value
whereas _space_group.crystal_system does not.  As the set of possible
values is different, these two data items are not strictly equivalent.
Therefore, _symmetry_cell_setting should be removed as an alias of
_space_group.crystal_system.

However, we cannot simply drop _symmetry_cell_setting. For better or worse,
there are over 800 CIF files in the COD which have _symmetry_cell_setting
as 'rhombohedral' instead of the recommended '_space_group_crystal_system'
of 'trigonal' (one has '_space_group_crystal_system' of 'rhombohedral' as
well!).  Of these, over 100 were produced in the last 5 years, although the
deprecation has been in place since 2005.

In order to interpret these "legacy" files, we need to preserve a
definition for _symmetry_cell_setting somewhere.

I suggest that we create an additional definition in the DDLm dictionary as
below.  Note that
(1) I have deliberately not provided a new dataname in <category>.<object>
form, instead retaining the legacy dataname
(2) I have placed the dataname in the space_group category
(3) I have deliberately included the word deprecated in the object_id (this
is only relevant for dREL methods).

Please comment.  The period for comments will be 3 weeks, after which this
change will be accepted unless there are outstanding issues.

James.
======================

save_symmetry_cell_setting

_definition.id    '_symmetry_cell_setting'
_name.category_id 'space_group'
_name.object_id   'deprecated_cell_setting'
_description.text
;
     This dataname is superseded by _space_group.crystal_system and is
retained here only to aid in interpreting
     legacy archives. Do not use it.

     Original definition: The cell setting for this space group symmetry.
;

_type.purpose                           State
_type.source                            Assigned
_type.container                         Single
_type.contents                          Code
loop_
  _enumeration_set.state
         triclinic
         monoclinic
         orthorhombic
         tetragonal
         trigonal
         hexagonal
         cubic
         rhombohedral
save_


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