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FFdecsa
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First, you need to know how decsa works, study the source of a classical
implementation. Then you have to understand how things are done in
slicing mode. Read all the documentation and have a working classical
implementation to compare partial results. There are comments spread
around the code. Some things are difficult to understand without paper
notes; for example the matrix transpositions and meaning of array
indices.

Sorry, it is hard to understand and modify ...

... but it was harder to design and implement!!!