{-

Applies Luhn's algorithm for numeric ID verification:

The Luhn algorithm is used to verify the validity of numeric identifiers

commonly used for credit/debit card numbers, government IDs, IMEI numbers,

etc. Given a list of one-digit numbers, it processes them as follows:

1. From right to left, double the value of every other digit. If a

product is greater than 9, subtract 9 from that result.

2. Sum up all the digits (i.e., the results from step 1 and the given values

of the other digits)

3. If the result is evenly divisible by 10, the identifier is valid.

E.g., given the identifier consisting of the numbers [2,7,5,8]:

1. We start by doubling the value of every other number starting from the

right, getting [4,7,10,8]. Since 10 > 9, we subtract 9 from it,

giving us the list [4,7,1,8]

2. Sum up all the digits, giving us: 4+7+1+8 = 20

3. 20 is evenly divisible by 10, so the identifier is valid.

E.g., given the identifier consisting of the numbers [4,6,1,8,5,3,8]

1. Doubling every other value and subtracting 9 when needed gets us

[4,3,1,7,5,6,8]

2. Summing them gets us 34

3. 34 is not evenly divisible by 10, so the identifier is invalid.

Examples:

luhn [2,7,5,8]

=> True

luhn [4,3,1,7,5,6,8]

=> False

luhn [3,9,2,8,6,4,1,7,2,0,5,2]

=> True

-}

luhn :: [Int] -- numeric ID

-> Bool -- True if valid, False otherwise

luhn = undefined

in haskell