<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def rvon_mises(mu, kappa, size=None):
 """ Random von Mises variates. """

    # TODO: Just return straight from numpy after release 1.3
    return (np.random.mtrand.vonmises(
        mu, kappa, size) + np.pi) % (2. * np.pi) - np.pi