10 lines
246 B
Haskell
10 lines
246 B
Haskell
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action p x = if p x then succ x else x
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fenceM p q s [] = guard (q s) >> return []
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fenceM p q s (x:xs) = do
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(f,g) <- p
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ys <- fenceM p q (g s) xs
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return $ f x ys
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ncsubseq = fenceM [((:), action even), (flip const, action odd)] (>= 3) 0
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