How to generate a sequence like "0, 1, -1, 2, -2, 3, -3..." in Prolog?

Question

Prolog

It is necessary to generate elements of such a sequence "0, 1, -1, 2, -2, 3, -3 ...". I just got to this, but the result is crashing:

num(1).
num(X) :- num(Y), X is -Y, X < 0.
num(X) :- num(Y), X is -Y + 1, X > 0.

conclusion:

?- num(X).
X = 1 ;
X = -1 ;
ERROR: Out of local stack
   Exception: (1,763,389) num(_6002) ?

I ask for help, because I have already boiled.

Answer 1

Here the last answer explains why it doesn't work and shows solutions:
https://stackoverflow.com/questions/19979548/predi...