def _num_det_vars(det_string, seasons=0):
    """Gives the number of deterministic variables specified by det_string and
    seasons.

    Parameters
    ----------
    det_string : str {"nc", "co", "ci", "lo", "li"}
        * "nc" - no deterministic terms
        * "co" - constant outside the cointegration relation
        * "ci" - constant within the cointegration relation
        * "lo" - linear trend outside the cointegration relation
        * "li" - linear trend within the cointegration relation

        Combinations of these are possible (e.g. "cili" or "colo" for linear
        trend with intercept). See the docstring of the :class:`VECM`-class for
        more information.
    seasons : int
        Number of periods in a seasonal cycle.

    Returns
    -------
    num : int
        Number of deterministic terms and number dummy variables for seasonal
        terms.
    """
    num = 0
    if "ci" in det_string or "co" in det_string:
        num += 1
    if "li" in det_string or "lo" in det_string:
        num += 1
    if seasons > 0:
        num += seasons - 1
    return num