Consider a (d 1 k 1 )-RLL constraint that is contained in a (d 2 k 2 )-RLL constraint, where k 1 2d 1 and d 2 > 0, and x a codeword length q > k 2 . It is shown that whenever there exist block decodable encoders with codeword length q for those two constraints, there exist such encoders where one is a subgraph of the other furthermore, both encoders can be decoded by e s s e n tially the same decoder. Speci cally, a (d 1 k 1 )-RLL constrained word is decoded by rst using a block decoder of the

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... d 2 k 2 )-RLL encoder, and then applying a certain function to the output of that decoder.