Identifies the operation of collection of operations
               from category PDBX_STRUCT_OPER_LIST.
               Operation expressions may have the forms:
                (1)        the single operation 1
                (1,2,5)    the operations 1, 2, 5
                (1-4)      the operations 1,2,3 and 4
                (1,2)(3,4) the combinations of operations
                           3 and 4 followed by 1 and 2 (i.e.
                           the cartesian product of parenthetical
                           groups applied from right to left)