def regression(self, slope=None):
self._calculate_averages()<EOL>clock_model = base_regression(self.tree.root.Q, slope)<EOL>clock_model['<STR_LIT>'] = self.explained_variance()<EOL>return clock_model<EOL>
regress tip values against branch values

        Parameters
        ----------
        slope : None, optional
            if given, the slope isn't optimized

        Returns
        -------
        dict
            regression parameters