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Users have requested a way to describe data structures with pointers linking objects to other objects. E.g. the arbitrary link structure a typical *NIX file system image can contain, with its hard links.
Offsets to Data
Some data formats contain header information including a table of offsets in the file to later parts of the data. The ability to directly express offsets within the data (absolute, or relative to some anchor, such as the end of the table of offsets) would make describing these kinds of data files much more direct.