empty(U-V) :-
unify_with_occurs_check(U, V).
push(Queue, Value, NewQueue) :-
append_dl(Queue, [Value|X]-X, NewQueue).
% when queue is empty pop fails.
pop([X|V]-U, X, V-U) :-
\+empty([X|V]-U).
append_dl(X-Y, Y-Z, X-Z).