1. Woleński, F.: What is formal in formal semantics. Dialectica 3(58), 427–436 (2004)
2. Chomsky, N.: Three models for the description of language. IRE Trans. Inf. Theory 2(3), 113–122 (1956)
3. Han, Y.S., Ko, S.K., Ng, T., Salomaa, K.: Consensus string problem for multiple regular languages. In Proceedings of the 11th International Conference on Language and Automata Theory and Applications, pp. 196–207, Springer, Sweden (2017)
4. Tian, J., Shao, Y., Zhao, X.Z.: Out subword-free languages and its subclasses. Int. J. Found. Comput. Sci. 3(27), 305–326 (2016)
5. Ito, M., Jürgensen, H., Shyr, H.J.: Outfix and infix codes and related classes of languages. J. Comput. Syst. Sci. 43, 484–508 (1991)