Radensky, A., and M. Djambazova. “Schemes for Recursion Elimination in Backus’ FP-Systems”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 83, no. 1, Dec. 1992, pp. 39-49, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/484.