Not sure whether it helped

With that change, we now have the following for noTwos and associated functions. (Oh, yeah... we made the one-digit case special as we did for counting the d-digit Ys, for the same reason, i.e. putting the special case in one place.)

evenNoTwos = [justOnes n (min 3 (n `div` 2 - 1)) | n <- [2,4..]]

noTwos :: Int -> [Integer]

noTwos n
    | even n     = base
    | otherwise  = concat [[p, p + tenToThe halfN] | p <- map spread base]
    where halfN    = n `div` 2
          base     = evenNoTwos !! (halfN - 1)
          spread x = let (a, b) = x `divMod` (tenToThe halfN)
                     in a * tenToThe (halfN + 1) + b


Did it make a difference? Yes, but... it was a bit slower. By profiling output, 193 ticks instead of 187; time output looked worse, with the best apparently

real    0m0.232s
user    0m0.200s
sys     0m0.028s

Recall that for the previous version, the best was

real    0m0.224s
user    0m0.176s
sys     0m0.044s

On the other hand, looking deeper into the profiler output, before, noTwos and things it called was eating 19.3% of the CPU time, while now that's 7.8%, but to be fair we should include evenNoTwos, which snarfs 7.3%--but still that's a total of 15.1%, which should be a win. Despite total allocation being down, actual memory usage is up, from maybe 6.25 MB to close to 7 MB; we are, after all, keeping more stuff around.

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