s(1).
s(2):- !.
s(3).

a(X) :- s(X).
% a) | ?- s(X).
% X = 1
% X = 2

b(X, Y) :- s(X), s(Y).
% b) | ?- s(X), s(Y).
% X = 1, Y = 1
% X = 1, Y = 2
% X = 2, Y = 1
% X = 2, Y = 2

c(X, Y) :- s(X), !, s(Y).
% c) | ?- s(X), !, s(Y).
% X = 1, Y = 1
% X = 2, Y = 2
