local

    fun estrai (entra(_, b)) = b
        | estrai (esce(_, b)) = b;
    (* oppure
        fun estrai el = case el of
              (entra(_, b)) => b
            | (esce(_, b)) => b;
     *)


    fun isOrdinata [] = true
      | isOrdinata [e] = true
      | isOrdinata(m::n::ns) =
            if(estrai(m) < estrai(n)) then
                isOrdinata(n::ns)
            else false;

     fun sort [] = []
        | sort [x] = [x]
        | sort(x :: y :: ys) =
            if (isOrdinata(x::y::ys) = false) then
                if estrai(x) < estrai(y) then
                    sort(x :: sort(y::ys))
                else
                    sort(y :: sort(x::ys))
            else
                (x :: y :: ys);


in
    fun ordina list = sort(list);
end;