in submix.py [0:0]
    def _get_lambda_from_dist_pair(self, p, p_prime, p0):
        if self.lambda_solver == 'iteration':
            rtn =  self._lambda_solver_iteration(p, p_prime, p0)
        elif self.lambda_solver == 'bisection':
            rtn = self._lambda_solver_bisection(p, p_prime, p0)
        return rtn