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DFDL schemas involve some large and complex regular expressions. Even the most advanced regular expression languages lack convenient ability to define a given construct once and name it, and then reuse it by somehow referencing that name. This would dramatically ease construction of regular expressions, and it is simply basic software engineering that large and complex things need to be named and reused, not duplicated.
Graph of Nodes and Edges Data
Users have requested a way to describe data structures with pointers linking objects to other object. E.g. the arbitrary link structure a typical *NIX file system image can contain, with its hard links.