Transpose3 inspired by: http://bugsquash.blogspot.ca/2013/06/optimizing-with-help-of-fscheck.html

transpose3.fsx

let transpose3 = function
    | [||] -> [||]
    | a ->
        Array.maxBy Array.length a
        |> Array.Parallel.mapi (fun i _ ->
            Array.filter (fun js -> Array.length js > i) a
            |> Array.map (fun sub -> sub.[i])
        )