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+Even with a fold-based representation of numbers, and pred/equal/subtraction, some recursive functions are going to be out of our reach.
+
+Need a general method, where f(n) doesn't just depend on f(n-1) or (f(n-1),f(n-2),..). Fibonacci can be done without Y. 
+Ackermann function would need Y, but surely we can find some simpler demonstration?
 
 How to do with recursion with omega.