let adder1 l1 l2 = 

    l1 

    |> List.map(fun x -> l2 |> List.map(fun y -> y + x))

    |> List.concat

​

let adder2 l1 l2 = 

    l1 

    |> List.collect(fun x -> l2 |> List.map(fun y -> y + x))

​

let adder3 l1 l2 = 

    l1 

    |> List.fold(fun acc x -> acc @ (l2 |> List.map(fun y -> y + x))) []

​

let r = [0;10;20]

let r2 = [1..3]

​

adder1 r r2 |> printfn "%A"

adder2 r r2 |> printfn "%A"

adder3 r r2 |> printfn "%A"