Error in defining Ackermann in Coq

Question

I am trying to define the Ackermann-Peters function in Coq, and I\'m getting an error message that I don\'t understand. As you can see, I\'m packaging the arguments a,

Answer 1

I just tried your function with Coq 8.4, and the error is slightly different:

Error: Nested recursive function are not allowed with Function

I guess the inner call to ack is the problem, but I don't know why.

Hope this helps a bit, V.

PS: The usual way I define Ack is what wires wrote, with an inner fixpoint.