I was hoping that Fascicle 7 would come out, as that would be on Constraint Programming, and I think that's very similar to SAT-solvers (Fascicle 6) and the BDDs that he's already covered.

Then again: SAT-solvers are a big enough subject matter on their own that backtracking search + SAT-solvers is more than enough to fill a volume of its own. So choosing these two subjects as volume 4B makes sense.

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As a fan of constraint programming, I'd love to see Knuth's take on the subject. I hope he doesn't skip over Fasicle 7.