rectify the prior information equation with the current state of the
        parameter_data dataframe.  Equations that list fixed, tied or missing parameters
        are removed. This method is called during Pst.write()

def rectify_pi(self):
        """ rectify the prior information equation with the current state of the
        parameter_data dataframe.  Equations that list fixed, tied or missing parameters
        are removed. This method is called during Pst.write()

        """
        if self.prior_information.shape[0] == 0:
            return
        self._parse_pi_par_names()
        adj_names = self.adj_par_names
        def is_good(names):
            for n in names:
                if n not in adj_names:
                    return False
            return True
        keep_idx = self.prior_information.names.\
            apply(lambda x: is_good(x))
        self.prior_information = self.prior_information.loc[keep_idx,:]