urn default_rule

    def _prune_rule_obj(self, X, y, rule):
        &#34;&#34;&#34;
        objective function for prune rule routine
        eq 7 from Cohen &amp; Singer (1999)
        &#34;&#34;&#34;

        V_plus, V_minus = self._get_design_matrices(X, y, rule)
        C_R = self._sample_weight(V_plus, V_minus)
        return (1 - V_plus - V_minus) + V_plus * np.exp(-C_R) \
               + V_minus * np.exp(C_R)

    def _eq_5(self, X, y, rule):
        &#34;&#34;&#34;
        equation 5 from Cohen &a