Complete this function in Ocaml

(* Sort list l via mergesort

cmp is a function that compares two elements of list l. When cmp

returns true, its first argument comes first in the sorted lest.

When cmp returns false, its second argument comes first in the

sorted list. *)

let rec mergesort (cmp : 'a->'a->bool) (l:'a list) : 'a list =

(* TODO, replace [] *)

[]