trim0 = trim '0'
trim x = dropWhile (==x)

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trimSp = twice (reverse . trim ' ') 
twice f = f . f

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whichDay d k = toEnum (fromEnum d - 1 + k `mod` 7) :: Day

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n # k = mod n k == 0
facs n = filter (n #) [1..n]

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prf  n = filter (n #) (takeWhile (<= isqrt n) primes)

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longest str = [ w | w <- wds, length w == maximum (map length wds) ]
              where wds = words str

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data Day = Sun | Mon | Tue | Wed | Thu | Fri | Sat
           deriving (Eq, Ord, Enum, Show)

primes      :: Integral a => [a]
primes       = map head (iterate sieve [2..])
sieve (p:xs) = [ x | x<-xs, x `rem` p /= 0 ]

isqrt :: Integral a => a -> a
isqrt = floor . sqrt . fromIntegral