But I do not find how to set the values in sage at t=0 for the functions x_d(t) and x_f(t). Before I did set it with maxima with  maxima.eval("atvalue(x_d(t), t=0, 0)$atvalue('diff(x_d(t),t), t=0, 0)$"); maxima.eval("atvalue(x_f(t), t=0, 0)$atvalue('diff(x_f(t),t), t=0, 0)$");