21. The apparatus of claim 16 , wherein the data structure generated by the program code comprises a tree, the tree comprising at least one set, at least one group, and at least one block, wherein the tree is hierarchical such that each set includes at least one node and is associated with exactly one group, and each group includes at least one set and is associated with exactly one block, and each block includes at least one group.
22. The apparatus of claim 21 , wherein the tree includes a directory with a plurality of directory entries, and wherein each directory entry describes one group or describes one set.