def METHOD_NAME():
    """Test expected output from constant trend."""
    y = pd.Series(np.arange(30))
    fh = -np.arange(30)  # in-sample fh
    forecaster = PolynomialTrendForecaster(degree=1)
    y_pred = forecaster.fit(y).predict(fh)
    np.testing.assert_array_almost_equal(y, y_pred)