Space-group symbol defined by Hall.
Each component of the space-group name is separated by a
space or an underscore. The use of a space is strongly
recommended. The underscore is only retained because it
was used in old CIFs. It should not be
used in new CIFs.
_space_group.name_Hall uniquely defines the space group and
its reference to a particular coordinate system.
Ref: Hall, S. R. (1981). Acta Cryst. A37, 517-525; erratum
(1981), A37, 921.
[See also International Tables for Crystallography
Vol. B (2001), Chapter 1.4, Appendix 1.4.2.]