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)