start(0).
t(0,a,0). t(0,a,1).
t(0,[],2). t(1,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).
path(K,X):-t(K,[],N),path(N,X).