(*
I want to return a list of common elements shared between two lists.
In particular, I want the cartesian product of two lists, such that:

let x = [2; 4; 6; 8; 8; 10; 12]
let y = [-7; -8; 2; 2; 3; 4; 4; 8; 8; 8;]

join(x, y) = [2; 2; 4; 4; 8; 8; 8; 8; 8; 8]

The following code is close to what I want: it returns distinct
elements shared between two lists, such that join(x, y) = [2; 3; 4; 8; 9]

-- Juliet
*)

let join x y =
    let rec loop = function
        | [], y | y, [] -> y
        | (x::xs as x'), (y::ys as y') ->
            if x = y then y :: loop(xs, ys)
            elif x > y then loop(x', ys)
            else loop(xs, y')
    loop(x, y)