def _append_to_endog(endog, new_y):
    """Append to the endogenous array

    Parameters
    ----------
    endog : np.ndarray, shape=(n_samples, [1])
        The existing endogenous array

    new_y : np.ndarray, shape=(n_samples)
        The new endogenous array to append
    """
    return np.concatenate((endog, new_y)) if \
        endog.ndim == 1 else \
        np.concatenate((endog.ravel(), new_y))[:, np.newaxis]