start(0).
t(0,a,1). t(0,b,2).
t(1,a,2). t(1,b,1).
t(2,a,2). t(2,b,2).
final(1).
accept(S):-start(I),path(I,S).
path(K,[_]):-final(K).
path(K,[H|T]):-t(K,H,N),path(N,T).