

1.  

?- a(X).
X = 1 ;
X = 2.

2.

?- a(X).
X = 1 ;
X = 1.

3.

?- a(X).
X = 1 ;
X = 2.

4.

?- a(X).
X = 1 ;
X = 1 ;
X = 2 ;
X = 2.

5.

?- a(X).
X = 3.
