def select(l : List[T], pred : T => Boolean): Option[(T, List[T])] = {
        type Discarded = List[T]
        type Remainings = List[T]
        def aux : (Discarded, Remainings) => Option[(T, List[T])] = {
               case (_, List()) => None
               case (ds, c :: tl) =>
                         if(pred(c)) Some((c, ds ::: tl ))
                         else aux(c :: ds, tl)
        }
        aux(List(), l)
}