% reach.pl, uses table modes as supported by B-Prolog version 7.3
% by Neng-Fa Zhou, April 4, 2009
% http://www.cs.kuleuven.be/~dtai/events/ASP-competition/Benchmarks/Reachability.shtml

:-include(base).

solve(As):-
    assert_facts(As), 
    query(X0,Y0),    % a given fact in As
    reach(X0,Y0,P),!,
    output(P),
    abolish_all.
solve(_):-
    format("UNSATISFIABLE~n"),
    abolish_all.

:-table reach(+,+,-).
reach(X,Y,[(X,Y)]):-
    edge(X,Y).
reach(X,Y,[(X,Z)|P]):-
    edge(X,Z),
    reach(Z,Y,P).

output([]).
output([(X,Y)|P]):-
    format("reaches(~w,~w). ",[X,Y]),
    output(P).
    
test:-
    solve([edge(1,3),edge(3,4),edge(3,5),edge(4,2),edge(2,5),query(1,2)]).