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Every K Elements

Implement the function 

val every_k : int -> 'a list -> 'a list

so that every_k k l is every \(k\)th element of l, starting with the first element. The behavior of the function is undefined if k is not positive.

Examples

let _ = assert (every_k 1 [1;2;3;4;5] = [1;2;3;4;5])
let _ = assert (every_k 2 [1;2;3;4;5] = [1;3;5])
let _ = assert (every_k 3 [1;2;3;4;5] = [1;4])
let _ = assert (every_k 4 [1;2;3;4;5] = [1;5])
let _ = assert (every_k 5 [1;2;3;4;5] = [1])