@Test
public void testLinearProcess() {

    SarimaSpecification spec = new SarimaSpecification(12);
    spec.airline();
    spec.setBP(1);
    SarimaModel arima = new SarimaModel(spec);
    arima.setBPhi(1, -.9);
    FastArimaForecasts fcasts = new FastArimaForecasts(arima, false);
    int N=12;
    double[] f1 = fcasts.forecasts(x1, N);
    double[] f2 = fcasts.forecasts(x2, N);
    double[] f3 = fcasts.forecasts(x3, N);
    for (int i = 0; i < N; ++i) {
        assertEquals(f3[i], f1[i] + f2[i], 1e-9);
    }
     N=60;
    f1 = fcasts.forecasts(x1, N);
    f2 = fcasts.forecasts(x2, N);
    f3 = fcasts.forecasts(x3, N);
    for (int i = 0; i < N; ++i) {
        assertEquals(f3[i], f1[i] + f2[i], 1e-9);
    }
}