let y1 = [1; 2; 3; 4; 5]
    let y2 = ['a'; 'b'; 'c'; 'd'; 'e']
    let rec Comb = function
        a1::b1 a2::b2-> a1::b1::Comb(b1 b2)
    printfn "%A" (Comb y1 y2)