Data items in the PDBX_STRUCT_ASYM_GEN category record details about
               the generation of the crystallographic asymmetric unit. The
               PDBX_STRUCT_ASYM_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_asym_gen.asym_id
    _pdbx_struct_asym_gen.entity_inst_id
    _pdbx_struct_asym_gen.oper_expression
      A  A  1
      B  B  1