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A general description is here.
Changes to S4 and S3 method dispatch will support S4 class inheritance for S3 methods, for primitives and via UseMethod() and do better on S3 inheritance per-instance for S4 methods. , if the argument S3methods=TRUE is given to setClass(). S4 method dispatch will use S3 per-object inheritance if S3Class() is set on the object. See ?Methods and the paper referenced there.