let rec filterOuter xs element =
    let rec filterInner xs element =
        match xs with
            []                      -> false
        |   h::_ when h = element   -> true
        |   _::t                    -> filterInner t element
    in
    if xs = [] then []
    else if filterInner (List.hd xs) element
    then List.hd xs :: filterOuter (List.tl xs) element
    else
        filterOuter (List.tl xs) element;;