Data items in the PDBX_STRUCT_MSYM_GEN category record details about
               the generation of the minimal asymmetric unit. For instance, this
               category can be used to provide this information for helical and point
               symmetry systems.   The PDBX_STRUCT_MSYM_GEN data items provide the
               specifications of the components that constitute the asymmetric unit
               in terms of cartesian transformations of deposited coordinates.
 
    loop_
    _pdbx_struct_msym_gen.msym_id
    _pdbx_struct_msym_gen.entity_inst_id
    _pdbx_struct_msym_gen.oper_expression
      A  A   3
      B  B   4
      C  B   5