% Source: http://www.cs.kuleuven.ac.be/~dtai/prototypes/dppd/petri-object.html
unsafe(_,_,s(s(X)),_,_).
unsafe(s(X),s(P),CS,Y,C) :- unsafe(X,P,s(CS),Y,C).
unsafe(X,P,s(CS),Y,C) :- unsafe(X,s(P),CS,s(Y),C).
unsafe(X,P,CS,s(Y),C) :- unsafe(s(X),P,CS,Y,s(C)).