write Ocaml programe to add all numbers in a list ?

the solution :

let rec sum list =

match list with

[ ] -> 0

| x:: tail -> x + (sum tail);;

let a = [2;4;5;6;1;9;10 ];;

sum a;;

question is :-Write a Ocaml Program to multiply each element in a list by 2. a= [2;4;5;6;1;9;10 ]