def METHOD_NAME(posterior):
    """Getitem performs as expected."""
    np.testing.assert_allclose(posterior[0].mean, posterior.states[0].mean)
    np.testing.assert_allclose(posterior[0].cov, posterior.states[0].cov)
    np.testing.assert_allclose(posterior[-1].mean, posterior.states[-1].mean)
    np.testing.assert_allclose(posterior[-1].cov, posterior.states[-1].cov)
    np.testing.assert_allclose(posterior[:].mean, posterior.states[:].mean)
    np.testing.assert_allclose(posterior[:].cov, posterior.states[:].cov)