checkCIF procedure

PLAT125

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PLAT125 Type_4 Test for ? _symmetry_space_group_name_Hall

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 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 outthe
 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