code: def less_than_pi_constraints(self):
          
  
          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
docs: 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