Space group symbol defined by Hall (Acta Cryst. (1981) A37,
               517-525) (See also International Tables for Crystallography
               Vol.B (1993) 1.4 Appendix B). 
               Each component of the space group name is separated by a 
               space or an underscore.  The use of space is strongly
               recommended.  The underscore is only retained because it
               was used in earlier archived files.  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.