let i arth n a r=
    if n<= 0 then 
        invalid_arg "invalide arg"
    else
        let rec suite n a =
            if n=0 then
                []
            else
                a::suite(n-1)(a+2)
        in suite n a;;

let rec concat l l2 = 
    match l with
    []->l2
    |e::l->e::concat l l2;;