Added subfolder structure

Homework 1/luhn.lhs

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+This is an implementation of the Luhn Algorithm. It's used to validate
+credit card numbers.
+
+Return the last digit of an integer number
+
+> lastDigit :: Integer -> Integer
+> lastDigit n = mod n 10
+
+Remove the last digit of an integer number and return the rest
+
+> dropLastDigit :: Integer -> Integer
+> dropLastDigit n = div n 10
+
+Put all digits of an integer number in reverse order into a list
+
+> toRevDigits :: Integer -> [Integer]
+> toRevDigits n
+>   | n < 1     = []
+>   | otherwise = lastDigit(n) : toRevDigits(dropLastDigit(n))
+
+Put all digits of an integer number into a list
+
+> toDigits :: Integer -> [Integer]
+> toDigits n = reverse(toRevDigits(n))
+
+Double every second value in the input list.
+
+> doubleEveryOther :: [Integer] -> [Integer]
+> doubleEveryOther([]) = []
+> doubleEveryOther(x:[]) = [x]
+> doubleEveryOther(a:(b:xs)) = a : 2*b : doubleEveryOther(xs)
+
+Sum up all digits from all integers.
+
+> sumDigits :: [Integer] -> Integer
+> sumDigits([]) = 0
+> sumDigits(0:xs) = sumDigits(xs)
+> sumDigits(x:xs) = lastDigit(x) + sumDigits(dropLastDigit(x) : xs)
+
+Evaluate if a credit card number is 'valid'.
+
+> luhn :: Integer -> Bool
+> luhn x = lastDigit(sumDigits(doubleEveryOther(toRevDigits(x)))) == 0