I remember my first steps of Prolog, I was following an exercise to define positive integer as a recursion of the same function ... I did not really known why I did that, but it was fun to define the integers (the funniest Prolog exercise is still to write a Prolog interpretor in Prolog). Today I'm reading SICP and I just found in Ex2.6 that the recursive integer definition is the basic of lambda calculus: Church' numerals! Definitively, I have to learn this lambda calculus. At least I have now a better understanding of it (thanks also to this post) and I wrote the addition and multiplication operators in Erlang.