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R6RS chose a nice set of features – single inheritance plus the ability to disable it, opacity, nongenerativity, procedural inspection and type creation – and gave them a very nice syntactic interface and a not-much-worse procedural interface. Its design is easy for an implementation to provide with good efficiency, and it’s formally absolutely correct, even preserving encapsulation under subtyping in a nice way.

Unfortunately, attempts to add other features to it invariably run into problems.