Problem 2. You will now implement a more efficient version of computing Tetranacci numbers in OCAML

(a) Implement an OCaml function sum top 4 : int list -> int list, which takes a list of integers and adds the sum of the top four elements to the head of the list (e.g. the input list [1;1;1;1;3;2] should become the output list [4;1;1;1;1;3;2]).

starter code:

let sum_top_4 (xs : int list) : int list = (* your work here *)