Ocaml: Write a function that sums 2 natrual numbers as represented by a list of integers between 0 and 9 where the head is the least signifigant digit.

Your function should be tail recursive

sum [4; 3; 2; 1] [1;0;1] = [5; 3; 3; 1]

sum [1] [9;9;9] = [0; 0; 0; 1]

sum [] [] = []

sum (nines 1000000) [1] does not stack overflow, when (nines 1000000) provides a list of 1000000 9s

*)

let rec sum (a : int list) (b : int list) : int list = failwith "unimplemented"