Restore the default value to the _geom_*.site_symmetry_* items

[vaitkus]

During the discussion in #58 it was decided to remove almost all default enumeration values due to them being potentially misleading. However, I would like to propose to restore the default value of 1_555 to the _geom_*.site_symmetry_* data items (e.g. _geom_angle.site_symmetry_1, _geom_hbond.site_symmetry_A, _geom_torsion.site_symmetry_3).

This value indicates that no translation nor symmetry operations were applied and therefore seems like a natural default (it matches the "I know nothing about the symmetry so I will do nothing about the symmetry" approach). Furthemore, data provided in geometry loops is almost always supplementary (i.e. derivable from the coordinates) and thus incorrect assignment of the default does not lead to a serious misinterpretation of the data (or at least to an easily detectable misinterpretation). There are at least two more good reasons to restore the default.

Firstly, there is a well established practice by some popular crystallographic software programs to omit the site symmetry item if all of the looped values are the default value (this practice is probably still ongoing). As a result, the removal of the default values from the dictionary made a large corpus of legacy CIF files a bit more ambiguous when interpreted under DDLm (missing is no longer equal to 1_555).

Secondly, since the site symmetry items serve as category loop keys and have the Encode purpose, the missing key values cannot be automatically elided during the validation process. Having a default values would remediate this.

I understand that the decision on the removal of default values might be final, but I just had to ask before making any sweeping changes to the legacy files (e.g. introducing site symmetry items populated with the default 1_555 values).

[jamesrhester]

I think you make reasonable arguments. I would like to run this past the core CIF group, just for good form as I doubt there will be any objections.

[vaitkus]

The core CIF group discussion can be found at https://www.iucr.org/__data/iucr/lists/coredmg/msg00460.html.

[vaitkus]

Can this be considered accepted given that there were no objections from core CIF group for over 2 months?

[jamesrhester]

Yes.