Fairouz Kamareddine Twan Laan Rob Nederpelt Revisiting the notion of function. 65-107 2003 54 J. Log. Algebr. Program. 1-2 http://dx.doi.org/10.1016/S1567-8326(02)00016-4 db/journals/jlp/jlp54.html#KamareddineLN03