prolog - multiple outputs, same variable -

how following program produce following output:

?- genn(3,r). r=0; r=1; r=2;

my current output r=0;

code:

genn(1,0). genn(n,r) :-      n > 1,      n1 n-1,      genn(n1,r1),     r r1.

newer code:

genn(1,r):-r 0. genn(n,r) :-      n > 1,      n1 n-1,      genn(n1,r),     r n1.

the above program meant output numbers 0 n-1.

if prolog implementation supports clpfd, write genn/2 this:

:- use_module(library(clpfd)).  genn(up,n) :-     n #>= 0,      n #<  up,     indomain(n).

sample query:

?- genn(3,n). n = 0 ; n = 1 ; n = 2.

Braziel