s(X,Y) :- X1 is X + 1, (Y is X1 ; s(X1,Y) ).

s1(X,Y) :- X1 is X + 1, Y is X + 1.
s1(X,Y) :- X1 is X + 1, s1(X1, Y).

s2(X,Y) :- X1 is X + 1, Y is X1.