def f(xx, uu, uuref, t, pp):
    """ Right hand side of the vectorfield defining the system dynamics

    :param xx:       state
    :param uu:       input
    :param uuref:    reference input (not used)
    :param t:        time (not used)
    :param pp:       additionial free parameters  (not used)

    :return:        xdot
    """
    x1, x2 = xx
    u1, = uu
    
    ff = [x2,
          u1]
    
    return ff