row([1,2,3]).
row([1,5,9]).
row([1,4,7]).
row([2,5,8]).
row([3,6,9]).
row([3,5,7]).
row([4,5,6]).
row([7,8,9]).

/*                                        General Utilities  */    
      
element_of(Head, [Head|_]).
element_of(X, [_|Tail]) :-
        element_of(X, Tail).

number_of_elements_of(1, [X]):- !.
number_of_elements_of(N, [_|X]):-
        number_of_elements_of(X),
        N is X+1.

append([], List, List).
append([X|L1], L2, [X|L3]) :- append(L1, L2, L3).