21.   Home water filtration systems exist in profusion.

22.   If the United Nations system did not exist, much of it would have to be invented.

23.   No such system existed previously.

24.   No such system exists anywhere, though the federal Bureau of Alcohol, Tobacco and Firearms is working to create one.

25.   No such system now exists.

26.   No nationwide system exists to track whether benefits have been received in other states.

27.   On the other hand, since some warning systems already exist, the V-chip debate can be seen as an excuse for superfluous political grandstanding.

28.   Our mental health system hardly exists.

29.   Service, however, is well below U.S. standards, and no centralized system exists for buying tickets.

30.   Senators received information about a week ago raising the possibility that such a system might exist, but they have not confirmed it.