Ramos, M. V. M., Almeida, J. C. B., Moreira, N., & de Queiroz, R. J. G. B. (2016). Formalization of the pumping lemma for context-free languages. Journal of Formalized Reasoning, 9(2), 53–68. https://doi.org/10.6092/issn.1972-5787/5595