test(2^N-T) :-
	length(_,N),
	N > 17,
	BN is 2 ^ N,
	length(L, BN),
	statistics(runtime,[T0|_]),
	t(L),
	statistics(runtime,[T1|_]),
	T is T1 - T0.

call_with_occurs_check(Goal) :-
	call_cleanup((set_prolog_flag(occurs_check,true), Goal), set_prolog_flag(occurs_check, false)).


t(L) :-
	unify_with_occurs_check(L,V),
	V \= [].