    else:

        Uprec = construct(n-1 , U)

        u, v, w = Uprec[0], Uprec[1], Uprec[2]

        u_next = u + v - w

        v_next = u - v + w

        w_next = (u - v) / w

        U += [ (u_next , v_next , w_next) ]

        return U[-1]



print( construct(7) )