checkCIF procedure

PLAT125

 Optionally specify the Hall symbol. The Hall symbol provides an
 unambiguous definition of the space-group symmetry where the Hermann-  
 Mauguin symbol leaves room for alternative choices of the origin.  
 E.g. for space-group P21, the screw axis is in general taken to coincide   
 with the b-axis. However, sometimes it is chosen to be shifted by 1/4  
 in the c-axis direction to bring out the relation with P21/c. The Hall 
 symbols will be 'P 2yb' and 'P 2ybc' respectively. 
 Refer to: S.R.Hall, Space-group Notation with an Explicit Origin;  
           Acta Cryst. (1981), A37, 517-525.
       or: http://www.kristall.ethz.ch/LFK/software/sginfo/hall_symbols.html