get the names of the prior information eqs that
        are listed as less than inequality constraints.  Zero-
        weighted pi are skipped

        Returns
        -------
        pandas.Series : pilbl of prior information that are non-zero weighted
                        less than constraints

def less_than_pi_constraints(self):
        """get the names of the prior information eqs that
        are listed as less than inequality constraints.  Zero-
        weighted pi are skipped

        Returns
        -------
        pandas.Series : pilbl of prior information that are non-zero weighted
                        less than constraints

        """

        pi = self.prior_information
        lt_pi = pi.loc[pi.apply(lambda x: self._is_less_const(x.obgnme) \
                                             and x.weight != 0.0, axis=1), "pilbl"]
        return lt_pi