*   Recursive diff validator

*   Callbacks for equal type cmp function.
    Provided function must return diff substructure with custom states.
    patch() also must provide callback interface to support such new states.
    profits: traditional text diff may be implemented for texts in leaf nodes and so on.

*   LCS for array's diff: use some digest (callback?) to calculate list's contents (or signature() from Data::Structure::Util)???

*   dstatus($diff, $path)
