def __init__(self, n_electrons=None, s=None):
        """NKG LDF setup

        :param n_electrons: Shower size (number of electrons).
        :param s: Shower age parameter.

        """
        if n_electrons is not None:
            self._n_electrons = n_electrons
        if s is not None:
            self._s = s

        self._cache_c_s_value()