Hongyan Xing Daowen Qiu Pumping lemma in context-free grammar theory based on complete residuated lattice-valued logic. 1141-1151 2009 160 Fuzzy Sets and Systems 8 http://dx.doi.org/10.1016/j.fss.2008.06.016 db/journals/fss/fss160.html#XingQ09