loop_
    _space_group_symop.id
    _space_group_symop.operation_xyz
    _space_group_symop.operation_description
  1    x,y,z          'identity mapping'
  2    -x,-y,-z       'inversion'
  3    -x,1/2+y,1/2-z '2-fold screw rotation with axis in (0,y,1/4)'
  4    x,1/2-y,1/2+z  'c glide reflection through the plane (x,1/4,y)'