Sum at K

Implement the function

val sum_at_k : int list -> int -> int

so that sum_at_k l k is the result of adding together every \(k\)^th element of l, starting from the 0-th element. The behavior of the function is undefined if k is not positive. Your implementation must be tail recursive.

let _ = assert (sum_at_k [1;2;3;4;5] 2 = 1 + 3 + 5)
let _ = assert (sum_at_k [1;2;3;4;5] 1 = 1 + 2 + 3 + 4 + 5)
let _ = assert (sum_at_k [1;2;3;4;5] 10 = 1)